Welcome to Simviation Fossilbot

Do you have FSX Acceleration?
If so, it comes with the FSX SDK which allows you to create missions and place scenery objects.
It's on your FSX Acceleration install disk/s.
If you don't have Acceleration, or if you want to design an airport from scratch rather than just adding objects to existing airports, try the freeware program ADE9 (Airport Design Editor).....just google it, it's available on many sites

Chuck