If I rememer right, you need to complete the Archives quest (and deal with Ursula one way or the other) before Thaddeus will appear and you will gain access to his room.
This can be done two ways - either save all three vegetables (to get the passwords for the individual rooms in order to lower the barriers one by one), or find the clue notes scattered around the city to get the code for the door. You've already found two of the vegetable guys, so you should probably concentrate on finding the third...