Garmin has a program with a name different from MapSource that's a native Mac app. MapInstall? MapManager? RoadTrip?

The difference is that it takes the official app maybe an hour to 'install' the map. If you drop by that site I mentioned, you can find out how to rename the map to an acceptable name and just drag it to the external memory card in a card reader. Voila! It's installed in seconds.

Garmin's come a long way lately in providing Mac support. See their BaseCamp, too.