Instructions
Navigate to a flight simulator software downloads site, such as flightsim.com (see Resources). Enter the plane that you are looking for in the "Text Input" box in the search. A list of plane downloads will be displayed. Click the "Download" button under the plane name that you want to download. Be sure that download is for FSX.
Click on the Windows "Start" menu button. Click on the "Downloads" menu. Click on the file you just downloaded to unzip it and open the folder in a new window.
Open "C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes" in another window.
Drag the folder for the aircraft you wish to install into the window you opened "C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes" in. The plane will now show up on the list of aircraft in the game.