Serial numbers are only used for Gift Certificates and are not needed for a normal Digital Map Store purchase.
The purchased maps are automatically registered in your account, and you just have to sign in from your device to access them. No serial number is needed.
When a user contacts us saying the app is prompting them to enter a serial number, it usually means they are trying to open the wrong map. It is best to delete the map and start over. See How to access purchased maps.