You can load through IMG2GPS as well, does the same thing, remember that existing maps you may have loaded will be erased.
The procedure after downloading IMG2GPS is to open the application, click on the load folder button, at which point a dialog box will ask you to select the maps (from whatever folder you have stored the maps).
In the next screen you check whatever you want to load (there is a list which will open up).
Click upload to GPS and you should see a small window open up telling you the progress of the upload. When it's done, the maps will be on the GPS
In case nothing loads, check to see if you have duplication within mapsets etc, should usually not get this problem.
About the Garmin maps, pester their tech help, which, BTW is not the greatest.