Well that answer is much better! ;D

For airports you can use AFCAD (somewhere at simV, make sure you get the most recent version),
it is designed for FS9, don't know if there's an FSX version, but don't believe so.
Note that you can't make all things linke exclusion rectangles with it.
For FSX and without some functions also for FS9 you can read some reviews about Aiprort Faciliator X (just google!), it is payware.
This two progs are only for airport layout (runway, taxiway, lights, parking places...), not for 3D buildings like tower, hangars, etc.