It seems atesterman was unclear...
1.Find xfire_games.ini
2.Open it and add the stuff he says in the link above. The code should be prefaced with [X + 1] where X is the number of the title-thing of the blob before it.
3.Add Trem to your installed games.
4.Watch all your Xfire friends wonder what Tremulous is.
5.Figure out how to add an icon.
