Instructions
Close "Flight Simulator X" (if it is currently running). You cannot install add-ons into the game if the game is in use by your operating system.
Click "Start" on your desktop. Open "Computer."
Open the "C" icon. Select "Program Files."
Open "Microsoft Games." Double-click the icon labeled "Microsoft Flight Simulator X."
Open the "SimObjects" folder in the "Microsoft Flight Simulator X" directory. Double-click the folder labeled "Airplanes." Drag the folder for any airplane add-on you've downloaded into the "Airplanes" folder. When the files are copied to the appropriate folder, the airplane add-ons will be installed and immediately be available in the game.