In case whell thrid button doesn't work as scroller here is solution:
(at least works for me).

Section "InputDevice"
        Identifier  "Mouse0"
        Driver      "mouse"
        Option      "Protocol" "auto"
        Option      "Device" "/dev/sysmouse"
        Option      "Buttons" "5"
        Option      "ZAxisMapping" "4 5"
EndSection