**broken link removed**
explains the connections for any PS/2 device, including power and ground.
Making a counter by using the µC already in the mouse would be a big way around. The µC doesn't send pulses or something back to the pc, but formatted data packets using the PS/2 protocol (also explained on the above site). So you would need some kind of second µC to decode the PS/2 information sent by the µC in the mouse. It would be far less work to just remove the original µC and replace it with a pic and do the counting directly.
Unless, of course, you can reprogram the µC of the mouse, but like i said before, they mostly use OTP devices.