the file xorg.conf is located in /etc/X11 or in the older systems in /etc/XFree86 (or something like it starting with a capital X anyway)
MAKE A BACKUP OF THE xorg.conf or XFree86.conf file before you edit it!!!
The line you are looking for is
Option "ZAxisMapping" "4 5"
Which you should add in the "Input device" section (the mouse section). You should identify it by Option "Protocol" "PS/2"
MAKE A BACKUP OF THE xorg.conf or XFree86.conf file before you edit it!!!
Save your changes and restart the X server (CTRL+ALT+Backspace). And you should be off. If that doesn't work, change the Protocol from PS/2 to IMPS/2 and you should be off.
Good luck!