There is a way to look at your User.ini file and see/change what it is bound to. I'm not at a Mac right now, but I think it's something like, Aplications->View contents of UT2004->System look for User.ini and open it in TextEdit. I think what you will want to search for is 'Mutate UT2VoteMenu' Cut and paste it on to a key that you have access to. Save, and you're all set.
Again, I'm not at a Mac, but I believe that's how it's done on OSX. Back up your User.ini before making any changes.