the XF86Config file is that file that contains configuration options for your X server. The configuration contains options for your mouse, keyboard, monitor resolution, etc.; and it is usually found under the directory path /etc/X11.
If you cannot find it, you can try to find it by issuing the following command at a shell prompt:
find /etc -type f -name XF86Config -print
This should give you the location of where XF86Config is if it is under the /etc directory.
IF for any reason you cannot find it under the /etc directory, try to find it starting from the root (i.e. a complete search). That is, type the follwoing at a shell prompt:
find / -type f -name XF86Config -print
This proceedure might take a while, but it should definitely find it.
A couple of things to note. If you cannot find it after doing a complete search, then you did not install XFree86 correctly, and should try to re-install it. In addition, run all of the commands as root, so that you do not get a bunch of permission denies trying to traverse through directories that may contain the file.
IF you find it, then just look at my previous post, edit the file, commit the changes, and re-start the X-server.
GL
P.S. Just out of curiosity, what distribution are you using?