I'm doing the thieves guild quests and I'm at the part where I gotta find mercer. And well I'm stuck does anyone have a walk through for this part?

What is the quest name,there are two different quests where you need to find Mercer ?

Yah hin mindah do faal Zuwuth Deykel het : http://www.uesp.net/wiki/Skyrim:Skyrim

I forget the name of the quest. But it's the one where have go on to this Dwarven cave find him and kill him before he get away with falmers eyes.

The place you're looking for is Irkngthand. The exterior magically spawns an extra set of rope bridges when you activate this quest, so even if you've cleared it before, now's the time to head back in. Followers aren't welcome in there, though.

http://www.uesp.net/wiki/Skyrim:Irkngthand