While windows users have good alternatives to Garmin MapInstall - on OS x the only real alternative is to use mkgmap.jar to create gmapsupp.img files. You can install them by placing them into the C:ProgramDataGARMINmapsmtbCOUNTRY.gmap respectively C:ProgramDataGARMINmapsveloCOUNTRY.gmap and better read the OS x tutorial too: respectively for Velomap users here:
This free software for Mac OS X was originally produced by GARMIN Corporation. The application is categorized as System Tools. The most popular version among MapInstall for Mac users is 2.1.