Have you looked at this tutorial?
https://www.gpsfiledepot.com/tutorials/how-to-open-maps-in-garmin-basecamp/It is a bit out of date, but the procedure is still the same, your map should appear in the dropdown menu as shown here

If it doesn't appear, then the map you downloaded may suffer from a bug that many older gpsfiledepot maps have. When Windows 7 64bit was released, Microsoft made a change that prevented these old maps from working. The "kludge" to fix this is to simply install another map that doesn't have the same "bug". After doing that, the buggy maps should start working.
The following map is known to work for this. Just install it, you don't need to "use" it for anything. Be sure to quit Basecamp before you install, then start it up after installing. You should then see your other map in the dropdown menu.
https://www.gpsfiledepot.com/maps/view/53/There are other possible causes of a problem like this, but I would start here.