chmod -r u+w ~/Games/Tremulous (assuming you own the file)
chmod -r g+w ~/Games/Tremulous (if for some odd reason you don't own the file in your home directory)
If that doesn't work try running the installer as root (with sudo). You should be able to do so as a user since the install path is in ~, but if it is trying to place an executable in /usr/bin for instance, you'll need the additional privileges.