As Bruce448 said, you need to have your AppData folder showing in your <user Name=""> file (eg, like "DavePC"),the same place that where your Picture Folder, Dowbload Folder, etc, are.
The AppData file is normally hidden. To show this file, in both Vista and Win 7, the easiest pathway I've found is:
Control Panel -> Appearance and Personalization -> Folder Options -> Show Hidden Folders. Tick the Show Hidden Folders box.
The AppData folder should now be showing in your <user Name=""> <User Name>file.
The pathway to the fsx.cfg file is:
AppData -> Roaming -> Microsoft -> FSX -> fsx (the .cfg file).
Note there is a Text Document folder called "fsx.CFG" right next to the actual fsx.cfg file;
I think this just stores your default fsx.cfg settings, or maybe your previous fsx.cfg settings if you do any changes.
I might add that the fsx.cfg has a lot of settings and you could muck up your FSX install, although not permanently, if you don't know what you're doing (which certainly applies to me anyway). And everybody seems to have different suggestions for tweaking the .cfg file.
In the end, I just used Jesus Bote's downloadable tweaked fsx.cfg (you fill in the details for your particular PC), and found it offered useful and noticeable performance improvements on both my old and new machines, although not quite the night and day differences some people claim. I don't have the link immediately available, but it's somewhere here on SOH.
Hope that helps and that I've understood your question correctly
Thanks - Rob
PS: I don't think the GAC plays any role here, you can leave it on or off as you like. As I understand, it stops non-default background programs from running until you approve.</user></user>