The can't unlock maps message is caused by Windows restricting access to certain files that Garmin Mobile PC needs after a map update. You can solve this problem by running the application as an administrator, which will give you access to the required files. The steps below will tell Windows to automatically run Garmin Mobile PC as an administrator.
To always run Mobile PC as an administrator:
This will ensure that Mobile PC has access to all of the unlock files that it needs, and will allow you to use the updated maps.