I don't know what you bought at radio shack so I don't know if it will work but all of the AA planes I have been on use the cigarette lighter plug (I've never been on an A300, though, thank goodness). Empower is a smaller 4 connector plug designed specifically for the airline industry. It is used on UA, CO, and US planes. Adapters such as Targus' have the empower plug with a cigarette lighter adapter.

Did you buy an AC inverter from Radio Shack? I know that some people use these but, frankly, I think that the higher voltage of of these devices could be dangerous on an airliner. They are certainly not approved for use by the FAA.