Use the force Luke
To edit and save the XF86Config file you'll need root priv's.
Personally, I use sudo from my personal account to do almost all admin work. (man sudo )
Otherwise, log in as root and do the edit and save. Caution........ save a copy of the working XF86Config file! ( cp /etc/XF86Config /etc/XF86Config_1_21_2004.bck ) (date and indicate it is a backup ;-) )
Uh - if you need an editor try vi /etc/XF86Config . Do a man on vi - the most used text editor.