You didn't say you had a Mac, that is a rather important little detail. Mapsource only works on Windows. Do you have access to a Windows computer? If so, I would install Mapsource on it if possible.
I've never had an issue like you describe with MapInstall on my own Mac(s), but I am running Sierra. My only other thought would be to try some older versions of MapInstall and see if they work. There's an archive of old versions here
http://www.gawisp.com/perry/macintosh/Unfortunately, Garmin has discontinued work on Basecamp and MapInstall, so there won't be any updates and they are also 32-bit programs, which means they won't work in the next version of MacOS.
