I'm supposed to pay this guy in the thieves guild stronghold, and I have no idea where he is!

he'll show up in your guild hall. I think he shows up a little while after your thieve return, so for a while you wonder where he is.

To be more precise, he's in the part of the first level right above the stairs that you went down to find Mae'var.

Finally figured him out, the crouched black assasin type guy named Joster. I have an initiate dialouge problem that is very annoying :( Thanks guys :)

I have an initiate dialouge problem that is very annoying...You have installed the official patch, right?