I was trying to use OSM in Basecamp, the version downloaded from the MAS. I realized I needed MapManager, so I downloaded and installed that. But it didn't work. I noticed that the maps would be accessible in MapManager's window, but not in Basecamp, and both had different Finder locations for the maps. And no way would Basecamp show the maps by any means that were showing in MapManager's window. Uninstalled, reinstalled, same result. I then noticed that for PC folks the Windows version of Basecamp installed MapManager.
So I again uninstalled. Downloaded Basecamp from Garmin directly (just Basecamp). Sure enough, it installed Basecamp, MapManager, and MapInstall. Clicked a .gmapi file, it installed, and was usable in Basecamp.
TL;DR: Do NOT use the MAS Basecamp edition, download it direct from Garmin.