I own the DNX7360BT and unfortunatelly, you can only purchase throught Kenwood, even though it's a Garmin GPS.
Have you tried to call GARMIN? Maybe they can help, i was lucky cause i just recently purchased the unit and they had to update my map.
I have a kenwood also and i recently purchased the Garmin Navigator NT maps which cost me about half the price Kenwood want, and work no problems at all. My Unit uses a SD card, and the new maps from garmin come on a Micro SD card in a SD adapter so either format will work fine.