Turbo is right. The system software updater works great natively on the mac (Webupdater) but the aviation database updater is only Windows compatible. Run Windows in VMware or Parallels you should be able to run the updater and have access to the machine's USB ports and everything.
I hope Garmin gets their act together for native Mac support soon, as Mac sales continue to grow. I'm going to do my first database update soon, and find it absurd that I have to run Windows to do it.