Mebbeth's hut is located near the center of Ragpicker's Square. To get there, I think you have to go to the northwest corner of the northwest sector of the Hive.
If you're trying to become a Mage as fast as you can, bear in mind that Mebbeth will ask you to perform some quests that involve a lot of running back and forth. The next Mage trainer won't be available until you have progressed significantly in the game, so as I said, be nice to Mebbeth and she'll reward you.

And don't forget her when you've gotten what you need from her.
SPOILER
Dak'kon (who can join your party) has enough skill to help you switch from Mage to Fighter and back, but he cannot help you acquire extra proficiency points (since he uses the mental discipline of the
karach rather than pure physical skill), and he cannot help you become a Mage for the first time (since he uses Gith magic instead of human magic). It doesn't hurt to get to know your companions--they're all very special.
