I am actually interested in this answer as well.  My mobo is a dell with no PS/2 connectors.  I got a Betson Imperial 3" Trackball w/PS2 connector and a OptiPac, for a great price, figuring that I could hook it up to that encoder.  The wires coming out are for the buttons not the OptiPac.  Is there a way to hook this up to an encoder or am I out of luck?