There are several ways to do this. Please have a look into the subtopics. Principally you can:
1. Use Mapsource/Basecamp (under Windows). It is a bit slow but reliable, and low on resources.
2. Use mkgmap (cross-platform) -- this is the quickest possibility but needs lots of RAM if you want to have address search for the map enabled on your GPS.
3. Use Qlandkarte GT (cross-platform) - might be difficult to compile the GPS drivers for your system.
4. Convert the maps (using Windows) to Garmin Roadtrip for Mac OSx and use that instead of Mapsource.
5. Use "sendmap".