To get that working you need to add two lines to your xorg.conf file (or xf86config file depending on what X implementation you use) section dealing with your mouse input device:
Code:
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/mouse"
Option "Buttons" "5" # THIS LINE!!
Option "ZAxisMapping" "4 5" # AND THIS LINE!!