Break yourself into a command line (Ctrl-Alt-F1) and cd to /etc/X11 then run
less xorg.conf
You're looking for a section like this:
Code:
Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "ZAxisMapping" "4 5"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/input/mice"
EndSection
Check what your mouse is assigned to (in this example, /dev/input/mice) and report back.