If you have FS installed in the usual directory, either Program Files or Program Files (x86) [I forget which, but the issue probably happens in either place] some sort of security routine that was introduced with Vista and carries over to Win7 won't allow you to change any cfg file, or much of anything else, except through the program's user interface (and maybe not even that way.) So if you want to make any changes that aren't done via the UI, you just can't make them. This apparently affects any program installed to those directories; it's not just an FS issue.
The solution is to install FS into a different directory. You might make a disk partition just for FS, or just make a folder for it on the C drive. Then you should be able to fiddle around to your heart's content.
P.S. - get yourself a twist grip stick. Even though it's not the standard way of controlling the rudder in a real plane, it does allow three axis control, and it gets to feeling very natural very quickly. Without three axis control you'll never be able to slip into a landing in a crosswind, never learn to coordinate a turn, and the only plane you'll ever be able to fly with any degree of realism is an Ercoupe.
Of course, rudder pedals are more realistic, but they're also much more expensive. I think most of us find a twist grip stick sufficient.
