Hi David,
Welcome.
Check you have folder …your P3Dv5\Navigraph, if not create it.
In latest FMS Data Manager,
In Addon Mappings , press +Add, for Please select an addon… choose Flight1 Mustang/King Air B200, for Install into… choose User Defined folder, specifying …your P3Dv5\Navigraph.
Press Save.
In Addon List select Flight1 Mustang/King Air B200 and Update.
Please let us know how you get on.
Cheers
Ian