You probably just need to edit /etc/X11/XF86Config-4. Mine (a USB mouse) looks like so:
Code:
Section "InputDevice"
        Identifier      "Configured Mouse"
        Driver          "mouse"
        Option          "CorePointer"
        Option          "Device"                "/dev/input/mice"
        Option          "Protocol"              "ImPS/2"
        Option          "Emulate3Buttons"       "true"
        Option          "ZAxisMapping"          "4 5"
        Option          "SendCoreEvents"        "true"
EndSection
My guess is that you don't have /dev/input/mice