I've read elsewhere the right procedure.
Download the .img file you want. Save them to a USB thumb drive in a folder named Garmin as gmapsupp.img.
You do need to use Jetmouse to generate a code based on your map and unit ID to enter when it begins the update.
So download Jetmouse and create the key
It should load automatically.
Another idea: Is may be the map too big for your unit ? Try to load just US .img