You probably didn't fix the permissions.
you might have to make it executable to do that(adjust the paths to suit where it was installed i think *buntuu installs it somewhere stupid)
su -c "chmod +x /usr/local/games/tremulous/tremulous.x86"
or
sudo chmod +x /usr/local/games/tremulous/tremulous.x86
also when you run it you gotta make sure that the executable is ran in the install directory by changing the shortcuts working directory to the install directory