Let me start by saying, I have never designed anything for FS, did a map once for Battlefeild Vietnam.
What I want to do is add referance lines on the ground at Stead Field in Reno, for air racing.
It sounds simple to me, but I don't know what software I need, if I can do it, and were to find the defult file to copy the existing scenery.
One of these two, read them over carefully to see which you need.
And the MS SDK should help.