Answer:
It depends on what system you're using.
If it's a Mac, right-click on the icon, go to 'Show Package Contents', Contents, Resources, Data, Aircraft, and copy the aircraft folder into it.
For Windows, look for the same folder (Aircraft) and do the same. It should be in C:\Program Files\FlightGear\Data\Aircraft
In Linux, open Terminal and log in as root, navigate to /usr/share/Games/FlightGear/Aircraft, and copy the aircraft folder there.
Once you have done this, the aircraft should show up in your startup wizard.