Are you talking about the "NPC is busy"-bug.
It's an WinXP and Win2K related bug, that is solved with the latest patches from Bioware (
www.bioware.com - obviously).
You can also pickpocket the Golem for the Sewer key, but if you are experience the bug mentioned above, you should install the patch as it will be reappearing on a lot of quest related NPCs (Xzar, Mae'Var, enal Bloodscalp, Lord Jierdan, Lehtinan, ... actually almost everyone).
There is a (very annoying) workaround: You can quicksave and reload again until the dialogue works (This can take AGES sometime)