Quote from the Ultimarc site.
Buttons. (Optipac)
If required connect two buttons to the button connections as marked (ie Port 1 L=left button. Port 1 R=right button). For each button, one connection goes to one of the four button inputs, the other goes to any connection marked "GND". You can use your control-panel buttons which are already connected to an I-PAC board. Just wire them to both an I-PAC input and an Opti-Pac mouse button input.
BobA