Open the file /etc/X11/XF86Config-4 in a text editor and look at Section "InputDevice". This is mine (for a 5 button optical ps/2 mouse),
Code:
Section "InputDevice"
# Identifier and driver
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "ExplorerPS/2"
Option "Device" "/dev/psaux"
Option "Buttons" "7"
Option "ZAxisMapping" "6 7"
EndSection
Yours will not look the same but if you post what you have there now plus the details on what sort of mouse you have I'm sure we can point you in the right direction.