Right now, Tremulous does not officially support Game controllers, and its unlikely that the devs would go into the trouble of doing so. You could use programs like Xpadder (Google it!) that will make your controller buttons act as keyboard keys. I use a Logitech Controller and I got it for
$20, it works well with Xpadder.
Also, for the other people here, please don't go "Why use a controller if you have a good mouse?" because saying that and not answering the question, is bullshit.
Also, this question was asked by Fleash Eater awhile back, did you try searching the forums before asking? Well, nvm, I hoped that helped. Cheers.