What do you use to Design/ Edit Airports?

I'm no scen. designer myself, but fiddled around with one or two things a couple of times. So until someone more knowledgable comes along...

AFCAD2.21 by Lee Swordy is extremely easy to edit the afcad. SceneGenX, RWY12 Object placer, ExcBuilderV2 are some tools that alow you to create/edit scenery.

AFCAD2.21 is available from Avsim.
RWY12 O.P. from http://www.rwy12.com/
SceneGenX you'll most likely find with google, can't remember where I got it from, same for ExclBuilder v2. The latter is a very basic yet very useful for creating flattens. The first lets you do a lot of things, but will require a bit more practise to get the hang of.

*BIG NOTE: When getting errors parsing the XML, make sure your system uses a PERIOD (.) for decimal symbol, and not a comma. I've been banging my head through brick walls of frustration because all my code was correct, yet it would not compile. Spent days, literally, trying to find out what the problem was. The answer was NO WHERE to be found on the internet at that time. Turns out these BGL-compilers do not work on systems that have a non-US configuration.

For AFCAD2.21 you should have FSUIPC so that you can link FS to AFCAD2.21, making it much more easy to correctly align gates etc. to the scenery (all is explained in the tutorial/helpfiles)
FSUIPC available from www.schiratti.com/dowson.html if not already installed.