I installed all the mods from guide and everything works fine. So for my first IWD walkthrough I decided to follow [url=http://www.gamebanshee.com/icewinddale/strategies/charactercreation.php]this guide concerning character creation.

I created a 6 man fighter party all with 18/00 strength, which wasn't too hard with because I used the Disabled Minimum Stats tweak from the IWD Tweak Pack.

Now I am at the beginning of Chapter Two, all level 7 and for some reason I cannot start to dual class any of my characters. All the classes are greyed out. I read that I'm supposed to be able to dual class at level 2 or 3, but I can't. So what's going on here, and when can I, if at all start to raise stats?

IIRC you need 17 Int to dual class to a mage, 17 Wis to dual to a cleric and 17 Dex to dual to a Thief.
For thieves there should also be alignment restrictions.
Did you create all Lawful Good Fighters with Int and Wis as dump stats?

What races are your characters? If I recall, only Humans can dual class. Also, you need a 17 in the prime stat of the class you want to dual to. If either of those are the reason for your troubles, maybe Dalekeeper will solve your problem without having to start over again.

They are all chaotic good human fighters. I tried to get 18/00 and the remaining points put in DEX/END on all of them. So it's true that none of them has a 17 stat. But then again, it's not possible to raise the starting stats?

No, not in-game.
You need an editor like Dalekeeper for that.