To use Above the Timber's maps in a Macintosh, you must first download (link) and install Garmin Mapinstall and use it to send a map container file (gmapsupp.img) to your GPS or a SD card. Mapinstall has limited map viewing functionality, its primary function is map loading.

Alternately you can download Garmin Roadtrip. Here's a link. Roadtrip behaves just like Mapsource in that you can both view the maps and install the maps to any Garmin GPS that has a USB connection, not serial.

The download file is a Mac Zip file (TGZ), in most cases your computer will automatically unzip the file into a TAR file, if not you will have to manually unzip the TGZ file to proceed.

You are strongly encouraged to download the free demo version of the maps, and test the install before purchasing the full map set. More complete directions are here.