I've done the first quest involving planting that ring on that dark elf character, but the guy is now asking to meet me down in the bar tavern underground. The Ragged Flagon or something, is it? Anyhow, day or night, he *never* ends up there. He just stands and shouts in the way he does when he's creating a distraction in that first quest. Closest I've ever seen him is in the water having a little swim, just in front of the entrance to the sewers. This is stopping me doing one or two quests, and it's really screwing me over. Anyone else having this problem, or know how to fix it?

~setstage TG00 stage 200 should fix it by advancing you though to the next quest.
I have a strong dislike for Brynjolf and the Thieves Guild members in general.

Or just reload a previous save.