Mac's unix, yes. If indeed you don't have permissions to write the file, there are two ways of setting them, the get info window, and the terminal command. The file is probably at /Users/you/Library/Applicaiton Support/Tremulous/base/autogen.cfg, you can get info on it there
In terminal, the command is chmod a+rwx ~/Library/Applicaiton Support/Tremulous/base/autogen.cfg
this will give everyone read, write, and execute permissions for that file.
As other people have said, though, you're probably better off opening the file in TextEdit and fiddling around with it manually.