Ah, I'm sorry, I misread the county as country, not county. You are right, the county lines are not mapped out on the map.
Unfortunately I don't have any experience with kmz or kml file, but someone posted up some sample code on how they got this running:
google-maps-api-using-kml-files-from-my-maps/
There may also be other users on the forum that have experience with using these files that may be able to help you.