Being that it's an .AIR file, all you should have to do is place it in your F18 Folder. The path is as follows:
Microsoft/Microsoft Games/Microsoft Flight SimulatorX/Sim Objects/Aircraft (or Airplanes)/F18
It should ask you if you want to replace the current file. Click yes. Then you will have to exit and restart FSX for the change to take effect. Hope this helps.
-Jimi