Here is a somewhat recent thread with some suggestions on interfacing with Mac.
You'll see a post from me with a note about MacGPS Pro, by James Associates. Overall it works well, with the caveat noted in my post about multiple uploads/downloads to/from the GPS. It is not free, but it DOES work directly with the USB interface. I use it with my Garmin 60C. Another very nice feature is the ability to purchase topos by state, or for specific national parks. You can use them on your computer--very nice for overlaying track logs, but of course can't load the maps into the GPS. You can use the topos to set waypoints & routes and upload them (the waypoints & routes) to the GPS.
I was also hesitant to try another Mac program (don't recall which) because it looked like it was either not supported, or not being updated regularly. I have not used any of the other programs listed, but they appear to work for others.
Thanks for your time in giving me this very helpful information. I will certainly try it. Also, it won't be too much longer until Garmin has Mapsource for Mac. They said by the end of 2006 and we are almost halfway through the year. That will hopefully solve are map problems.