I've purchased a map (City Navigator Middle East & North Africa NT 2015 map) for my Montana 600 unit, and when I tried to install my it I couldn't unlock it. I've tried to do that through Garmin Basecamp with no luck.
This is the Garmin Express forum and is not the correct location for this posting. I will provide a suggestion though.
I see that the City Navigator Middle East & Northern Africa NT 2015.20 mapping was unlocked to this device in February 2015. You should be able to use the Unlock Online process in the following FAQ to unlock the maps for use in BaseCamp.
How do I unlock maps in BaseCamp?
If you continue to experience difficulties with BaseCamp, please use the correct forum or contact Product Support.