Are you trying to play GPP? This is a common problem when you install GPP after having played 1.1. You need to navigate to "/Users/[YOUR NAME]/Library/Application Support/" using finder. Once you're there rename the folder "Tremulous" to "Tremulous1.1".
GPP should now work.
Once you run GPP, it will create a new "Tremulous" folder in Application Support. If you want to go back to playing 1.1, rename this newly created "Tremulous" folder "TremulousGPP" and rename the old "Tremulous1.1" folder back to "Tremulous".