Im kinda stuck in the underdark, i want to find the mage who was imprisioned but can still be released, where must i cast the freedom spell to find this guy?

There's a sort of biggish open area just north of the merchants where he'll appear. However, doesn't it just affect anyone in the entire area, no matter where you cast it? I don't remember, as it has been awhile since I've played through that particular part.

It doesn't matter where you cast it, it affects the entire Underdark screen. I found that a little odd at first too.