I've never used an Ipac, so can't give you instructions on how to do this.

What you need to do is configure the Ipac to send the correct keystrokes that correspond to the Mach3 Hotkeys.
Most Mach3 buttons already have hotkeys assigned to them.
If you want to use the Ipac to activate a button with no default Hotkey, then you'll need to add the hotkey with a screen editor. Most people these days use eithe Machscreen or Screen4.

Links to those and the original Mach3 Screen Designer are here: Screensets - Newfangled Solutions