loading GDMBR-basemap-v1.img on BaseCamp for Mac

[fargoman]

I can't figure out how to load the Great Divide basemap (GDMBR-basemap-v1.img) into Garmin BaseCamp for Mac.  In BaseCamp, I've tried the File>Import command, but the 'GDMBR-basemap-v1.img' file is grayed out. Garmin tech support was no help in explaining why a purchased map (Garmin's Topo U.S. 24K West) can be imported to BaseCamp, but the GDMBR basemap can't, even though they both have a '.img' file extension.

I loaded the GDMBR basemap onto my Garmin 62sc's SD card successfully, which is where I really only NEED to have it (along with the TourDivide2012_v2_10k.gpx route file), but I would like to be able to see both the GDMBR route and basemap in BaseCamp on my Mac.  Any help would be greatly appreciated.  Thanks!