Thanks for the reply. I've tried everything you indicated, but both BaseCamp and MapSource insist the files are not recognizable.
Exactly what is Mapsource "saying" when it "insists" the maps are not recognizable? Again, you don't "open" those map files. You just run the installer, then start the program and choose the map from the dropdown menu.
Then, either it works or it doesn't. It will not "say" anything to you. If there was a problem, the map just won't appear on the menu. There won't be any error message. You could try running GMTK, it's a free utility program for managing Garmin maps. When it starts, it will do an integrity check on all your maps and try to automatically fix them. You could post any problems it reports here.http://www.javawa.nl/gmtk_en.html
What operating system are you using? Some maps here have an issue due to an old installer that prevents them from working on Windows 7 64 bit (possibly windows 8 also). The issue is related to the registry and it can be resolved by installing a map that creates the correct keys. If you are using one of these operating systems, quit from Mapsource/Basecamp, try installing this map, then open Mapsource again and see if fixes the problem.http://www.gpsfiledepot.com/maps/view/53
Aside from that, most of Garmin's devices don't include any usable maps, just a crude basemap. Garmin expects you to purchase one of their own maps and if you do, they will hold your hand until you get it working. When you come to a site like this, the process may not be as simple because the maps are free and the authors may have made some mistakes when they created them. But many thousands of people have gotten them to work, and with your 30 years of experience I'm sure you can too.
FYI - if you have a newer Garmin device, you will want to use Basecamp instead of Mapsource which is discontinued and does not fully support the newer models.