I have downloaded a PC12 package from the internet, i pasted it in HP (C>Program Files>Microsoft Games>Microsoft Flight Simulator X>Simobjects> Airplanes, and it didn't appear in the simulator plane choice section. I also noticed that a few other planes were in the Airplanes file, but not in the simulator such as the Dash 8. Could this be associated with the problem?
Be sure that the box "Show All Variations" is checked.
It's in the tab that you choose the aircraft.