If you were trying to quickly click through the discussion, it is possible that you told Aleister that you do not want to join the Guild of Mages, and not to ask you again (this has happened to me a few times).
There are a few ways to fix this:
1. Restart the game (drastic and not recommended).
2. Reload a previous save (easiest way, provided you haven't overwritten your previous save - I've always done this).
3. Use the game editor to find "02-Guild of Mages Member" under the "Quests" tab, and set it to "Not Started". (Make sure you read here:
http://www.gamebanshee.com/forums/dunge ... 72087.html to know how to avoid losing experience points.)
4. If 3 doesn't work, using the editor, find "01-Guild of Mages" under the "Quests" tab, and set it to "Started." Then, click the "Items" tab, and click "Add." In the window that pops up, type "0116" for the "Item Code," and make sure that "ID'd" is checked under "Item Stats." Then save. (This will give you the Staff of Scaris [Scorching], and activate the Guild of Mages initiation quest, which is to deliver the staff to Rianne in the Devil’s Marsh. No guarantees, but completing this quest may allow you to then continue to take quests from the Guild of Mages as usual.)