Is there a program that allows you to create/edit terrain that can work for CFS2? I want this because I need flat area for the airport I'm making.

As Hagar says, if you don't already use FSSC, which is available here:

http://fssc.avsim.net/

you should start. It is a wonderfully versatile tool that will allow you easily add flatten areas to the scenery. Just make sure that the scenery altitude and the flatten match or your runway may end up under the ground or floating above it.

For a more robust/sophisticated if at times confusing and frustrating terrain and scenery altering program try googling on a program called:

Ground2K V4.zip

I believe it and the tutorial that goes with it and the upddated bgls are all available over at AvSim.

However, if you are just getting your feet wet in scenery building, stick to FSSC. It should fill all of your needs for the time being.

There is terrific, very simple tutorial named How to Add A Base to CFS2 by John Appelgate, aka m4ilm4n. Available here:

http://www.geocities.com/m4ilm4n2001/

It is a straight forward easy to follow tutorial that leads you thru the process of making a base with FSSC.  I recommentd it.

Hope this helps,
Cheers,
Chris