You'll need to edit your XF86Config file. Do this as root:
Code:
mcedit /etc/X11/XF86Config
Under the
Code:
Section "InputDevice"
Identifier "MouseSomethingOrOther"
Driver "mouse"
Change 'Option "Protocol" "auto"' to
Code:
Option "Protocol" "IMPS/2"
and add
Code:
Option "ZAxisMapping" "4 5"
before EndSection
Restart X and the wheel should be working.