The files are .zip files, to install once downloaded, double-click in the Finder to produce a .gmapi file. If you now double-click the .gmapi it will run another Garmin app called MapManager which came with Bobcat from memory, which puts the maps in the right place for Bobcat to use them. If you don't have MapManager all it seems to do is copy the .gmapi file to the right place, which is
/User/"you_user_name"/Library/Application Support/Garmin/Maps
and rename the file to have a .gmap extension.