How do you find your Flight Simulator X Files
On you main hard drive, most likely C Drive, you should start looking for a folder named "Microsoft Flight Simulator X" or "Microsoft Flight Simulator X Files", something of the like. Then, you will find all of the files, for every detail of the game; aircraft, ground equipment, etc.
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If you want to install aircraft mods that you have downloaded, follow these instructions:
For previously downloaded files of airplanes, you must save them to a folder that you can locate, >C/Flight Simulator X/Airplane mods< for an example. You must then have main folder open, where all of the data for FSX is stored. (Every program makes its' own folder and puts all its' downloaded content there to access it later).
There should be a folder inside called >Aircraft< or something of the like. In here, you should find all the aircraft in the game. Now, open your other folder with the downloaded files in it. Select the files you want to copy over, and drag them into the folder. They should appear in the folder. Now, when you next boot the game, the planes should show up in the menu choices. If not, the files could be corrupt or not all there.
Hope this helps someone, of not you!