Instructions
Open "C:\Documents and Settings\Username\Application Data\Microsoft\FSX," where "Username" is the name you use to log in to Windows. Click "Tools" from the menu, then "Folder Options." Select the "View" tab and check the box next to "Show Hidden Files and Folders." Click "OK."
Open the file "fsx.cfg" in Notepad from the folder you opened in Step 1. Look for the line beginning "SimObjectPaths" and beneath it, enter "SimObjectsPath.6=Aircraft" on a blank line. Save and close the file.
Open up your "FS9" folder, which by default is "C:\Program Files\Microsoft Games\Flight Simulator 9." Right-click on the aircraft you wish to copy, for example "Lockheed_Vega," and click "Copy."
Open your "FSX" folder, which is by default "C:\Program Files\Microsoft Games\Microsoft Flight Simulator X." Right-click in the folder, select "New," and click "New Folder." Name the new folder "Aircraft" and press "Enter."
Open the new "Aircraft" folder. Right-click and click "Paste" to copy the aircraft to the new folder.
Run "Flight Simulator X" and click "Free Flight" from the main menu. Click "Select Aircraft" and click the airplane you added to fly it.