In my experience, airplanes will not land to unload cargo due to the way the AI behaves. I assume you want the plane to land, since you said you tried the Transport Unload waypoint, which is the only reason you'd use it. If you intended for them to parachute from the plane, you'd need to some code to accomplish that, not a waypoint.
As for a US cargo plane, Captain Moore made a C-130, which you can find somewhere out there on the web.