I assume this is a pre-loaded Garmin map card? If so, it must be on some kind of "removable media" for basecamp to use it. You could copy the whole card (not just the file) to a USB flash drive and that should work. If you don't want to use a physical device, use Disk Utility (in the Applications/Utilities folder) to create a disk image and copy the card contents to the image.
You will now have a .dmg file that will mount as a disk if you double-click it. Basecamp should be able to access the map this way.
