I think ADE is a famous one. http://www.airportdesigneditor.co.uk/ It uses the tools the FSX SDK delivers, so you have to run FSX Deluxe or Gold. Just install the SDK from your FSX discs and update it with the links on their site.
The rest should be in the manual and if it's just about frequencies and stuff, it should be easy.

Which airports do you have in mind by the way? As said, some folks may already offer the larger ones as freeware and ready to use. No tools needed.