Okay, so I thought I'd followed the instructions. I downloaded and installed my maps and can see they're installed (see first screen shot.) But when I go to Basecamp, it only shows that Global Maps are installed. Not sure what to do next.
I'd say that either you did something wrong or else there is a problem with the map you installed. Try installing a different map and see what happens. There's a problem with the Windows version of Basecamp, but AFAIK it does not affect the Mac version.
On the Mac, if everything is OK with the map, when you double-click the file then MapManager should open and ask if you want to install the map. It appears that this did not happen. If you want to try a quick test, install the Mac version of my own map here: https://boydsmaps.com/boyds-map-of-new-jersey-2018/
I know that this works properly, and it includes detailed instructions.
Yeah...sure 'nuf. There it is, along with the other two I'd attempted to install. I shut down Basecamp, restarted it, and they were available from the dropdown. Thanks for your help.
Great! If Basecamp was running when you installed the maps, then they probably wouldn't show until you quit and started again.