Sadly, I fear you will find that garmin are very a unMac company. I noticed recently that their new range on iQue PDA's are accessible to Macs but they have never previously supported Mac.
there is even a petition to get them to do so (Google Garmin Mac and you'll find it.)
There are 3rd party solutions available - try this
http://www.macgpspro.com/
not tried it myself, but plan to.
whatever else you plan to use your Mac for will be a joy though, i promise...