I stuck about a year ago with awesome Wizardry 8 game.
Now, I dicided to continue my old save, but do not know what to do.
I am deeper on tree houses area, and I am not able to move forward anywhere.
The only thing I know is that I missed somehow some special potion from NPC.
I remember that this house was a long way from my current location, and I am not sure which one that was, so I am not happy to turn back, fight all respawned nemies again, and check each of a lot of houses.
Is it possible to move forward without this special potion somehow?
Is there any code that can bring this item to my inventory?
At least, how to find this NPC very quickly if turning back is the only way?

The house with the NPC that sells the potion is near the start of the Trynton Upper Branches area. It isn't that far from where you are; in fact, you are probably still in the same area.

You don't really need to do the Potion thing to proceed. You just get a good bunch of xp and a lot of backstory, but you won't be stuck if you don't do it.

Well as far as I remember, the only ways to move forward were locked.
I moved around many times, and did not found any way to go somewhere except backward.
I read somewhere that I will not be able to unlock the way until I have special potion that is related with some quest.
Here is my last save, so if you are able to tell me what should I do there, then I would appreciate it: http://www.speedyshare.com/HwzFA/Saves.zip
If this potion is optional, then I do not care about extra XP and some back story if I need to go back for it.
If not a potion, then there must be something other that I missed.

In the first third of the game you cannot do everything there is to do in Trynton. You have to go back 1 or 2 times anyway later on. What you can do the first time you get there is more or less the following:

- solve a fountain riddle and get permanent +5 intelligence for everyone plus some extra reward
- get 2 short vines which can be combined to a vine rope
- clear out the Zoo and get a quest item and a gadget component
- visit the 7th bow with a Mystery Potion and a Zuzu petal
- use the vine rope to repair the bridge to the Rattkin tree
- destroy the Breeders and get quest rewards by Chief Gary (at the tree entry) and Madras (5th bow, I think)

There is some stuff you can only do later on, but nothing of the things mentioned above is really essential for the game (your later visit will be). You can just as well return to the ground and visit the Swamp or whatever.

Right now I'm not at home, but once I am I can try your savegame.

You are still in the correct area. Leave that hall where you are, down to the main way. Turn right and make your way through the Zoo and a bit further to Fuzzfas' Shop. There you can buy the "Mystery Potion".

You got a Ranger and thus don't need to search actively btw. Active searching gives the initiative to the opponents, should a fight occur. Rangers search anyway, without active searching. As is appropriate she also has the highest Senses and is better than your other guys at searching.
Also you should sell some junk or put it in a chest somewhere: you keep many useless things, not least (heavy!) armor you don't use (although some of that armor is actually better than the armor you use).

You recently met the one and only Gadgeteer one can recruit (Madras). He's not too popular but still an option - if you do his mission. Either way your party carries around a bunch of Gadgets: don't mistake those trinkets for quest relevant! They can be discerned by checking who can use them. Quest items can always be used by everybody and they don't require anything (no specific skills). Unless you hire that Gadgeteer those are but junk. You can dump stuff anywhere and return for it later, should you need it after all (e.g. if you hire Madras).

I've had a look at your save and things are not that bad.

if you do grab a gadgeteer then join the doll and box together.
the quest you are talking about is not aval yet because you don't have the potion as pointed out , you get it from the shop down one level but that won't help you yet because you don't have a statue from a quest in the swamp area (starts with the bug fort) so I would not worry about it yet.

I recommend you go UP to the next level (where the rats are) but instead of turning right, go ahead and down the lift... (go out side) this area has a rope ladder that makes it easy to return later on when you have the other items.

You have to do the swamp next, then the bug fort (you have options here to take a side or play both against each other) then the mines area north east of swamp opens a back door into the bug fort & THEN you can come back to finish the tree quests... even then its possible to miss items but thats the minimum.

Ok, the save is about 18 month old, and I must say that I forgot almost everything.
I do not remember the main plot, dialogues, maps that I visited, and tactics that I used in combat. I do not go with search turned on all the time.
I used it in this case because I was trying to find if I missed something.

Yes, a lot of various items, but after playing some RPG like games, I learned to keep unknown purpose items, because they are sometimes required later in the game (turn back the whole way to find them back? no way! and in some cases not possible).
Supply of ammo is also thing learned from some games, when you were in moment where were not able to buy or find more for longer time, so this is just in case to be prepared for such situation.
I guess that I was probably used ranged weapons before enemy could reach me.
Other items are probably collected for sale. I am in hall with four directions, and in front of me is entrance to garden room with sign named "7th" (the place where I should have special potion).
If there is nothing important to push main game forward, then I can leave without making side quests.
My question is, if I have entrance to garden with "7th" sign in front of me, where should I go exactly?
Left, right or backward?
How to reach the swamp from place where I am now?
Will I have to go back here again or is it optional only?

In any case you exit the hall. That's the door with no room behind. You'll get to a crossing. Two options:

Go right if you want the potion and/or leave Trynton the way you came. Make your way more or less straight, into some fights against fairies and wasps. Those aren't easy actually. I'd suggest equipping Potions of Cure Light Condition to cure blindness & Dracon Breath to use the breath special attack against the fairies. Let the Trynnie patrols fight for you and hope they die: they are really annoying blocking your way while alive. Proceed until you come accross a fountain next to a house with a dragon head inside.

Go straight @ the crossing to head upwards, the way ussnorway suggested. You'd want to proceed to the Rattkin tree. As mentioned there is a shortcut for leaving Trynton. Once on the Rattkin Tree you just follow the bridge and turn left after a while, where you'll see an elevator. Use it to go down.

Third option would be left, where you'll find Madras. Probably useless for the time being.

The "potion quest" is not a side quest: it is part of the main quest. That said you do not need it. It is only background story and an experience reward.
No matter what you do you'll have to return to Trynton.

Once you left Trynton you head to a path near the big tree that is Trynton. Go right to proceed to the swamp.

Ok, I will proceed to Rattkin tree, evelvator and then the swamp.
Thank you all for detailed descriptions.

This game is one of my favorite RPGs.
In practice, I did not seen any other game that would be similar to Wizardry 8.
I know about Magic and Magic series, and even played latest X Legacy, but tactical combat and character development are simplified, and also other aspects of these games are not immersive enough for me, and I feel that I am playing just mechanical game.
Wizardry 8 provokes my imagination somehow and creates illusion of real living world (even with old, but still cool graphics style), giving me also advanced tactical combat experience (I am a big fan of Jagged Alliance 2).