This doesn't have anything to do with the antivirus, and of course is not a problem of your download (a corrupted download will not install) it's a problem we are looking into right now, because while we are upgrading all our products in preparation for GSX (that will be replace ParkMe and will be free to use at our airports) and something must have affected the existing ParkMe.
Just use the scenery without it until we fix it, most likely with an updated Addon Manager installer.