If you go on the ipac website you can download the client which allows you to set any input on the ipac to any button (or combo) you like.

You just plug it into your PC, run the software and use the graphical interface to setup however you wish. The labels on the ipac pcb refer to the software so its easy to know which your using. There a full guide to configuring it in the ipac website, you just need to know what key in mach3 does what then its up to you which input in the pcb you assign to what function.

I've got a ipac2 which controls a modified skyHD box I've squeezed a PC into :)