Never used this tool, because there are no cards for my country with DEM information.
Isn't it an option to remove that requirement?
Move the "US Basemap v2.gmap" out of the standard maps folder and try again.
Is there any chance that BaseCamp will ever be compatible with existing street map products that do not have DEM.
ln -s /Users/you/Library/Application\ Support/Garmin/Maps/unsupported/somemap.gmap /Applications/Garmin\ BaseCamp.app/Contents/Resources/