I mastered Thief profession, died, and chose robber after that. How the hell do I get robber units???

Do you have a Fortress ;)

Nope. I guess I'm gonna have to send in my thieves with rapiers and pistols aren't I? Speaking of which, how the hell do you play a raid because I tried one, got into the fort, and was shown my men's health and the enemy's health. I watched my men die. WTF do I have to do!?
Oh, and TheJoe, I am as addicted as you are now. I cant quit playing (except for facebook, gf, and game making,)

Thieves are only for theiving and sabotage, Robbers are for waylaying carts and some other stuff.
You'll need a Robber's Fortress (there's some outside of the town), they can be bought like normal businesses.
I've never been a Robber, so I don't know quite how they work (the most I've ever been is a Priest, a Patron, a Thief and a Perfumer - yes that's one character).

At the sad cost of 3 of my best workers, I took over the enemy fortress and 5 of it's men now work for me. I learned my lesson...BRING MORE MEN!

Poor thing.

WEll, I didn't know how to fight and things got out of hand. I learned that all you have to do is occupy 2 of the 3 flags to win. Pretty cool if you ask me since I can just hire dumbasses and use them for sword/dagger/pistol fodder.

Once your robbers are good, you make DAMN good money by extorting for protection.. Nothing else seems to bring in as much in the time/manpower/risk ratio. (generally it seems about 1-2K per robber, if you have well trained journeymen.