Code:
Section "InputDevice"
Identifier "PS/2 Mouse"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "ZAxisMapping" "4 5"
Option "Device" "/dev/psaux"
Option "Emulate3Buttons" "true"
Option "Emulate3Timeout" "70"
Option "SendCoreEvents" "true"
EndSection
Option "ZAxisMapping" "4 5"
Add a similar line like this to that section in /etc/X11/XF86Config-4 or
/etc/X11/XF86Config, whichever is newer.
What mouse is it? How many buttons including wheel button?