What kind of scenery are you trying to make? Flight 1 has an airport editor you can use to edit and create airports. There's also Instant Scenery which can add objects like buildings, hangars and airport equipment inside the sim (WYSWYG) just like FSEnhancer for FS2004 from Lago. If you are talking about building scenery from scratch then you'll need 3D modeling software like GMax and other tools to put it into the sim.
I've been pretty happy with Instant Scenery. It's easy to work with and a lot less buggy than FSEnhancer and you can share files with people who don't have Instant Scenery. I did this last night for a little strip in Utah.
