@marion A very good point is made by @rbaker above. Those of us with arcade panels have built them differently, but we might also have our IPAC devices configured differently, so even if we did share config files, the hardware keys mapped inside the IPAC may not match.
The image above in rbaker's post shows the button layout as two rows of four buttons:1sw1 1sw2 1sw3 1sw4 1sw5 1sw6 1sw7 1sw8
while my panel looks like this:1sw1 1sw2 1sw3 1sw4 1sw5 1sw6
Furthermore, while rbaker uses LSHIFT for 1sw1, I am using LCONTROL. This is the keystroke that gets sent when I press my button wired to 1sw1 because that is how my IPAC is configured (i.e. using WinIPAC utility). The wiring matters, the location on your panel matters, and the actual key the IPAC is sending matters--and this is BEFORE any mapping is configured inside a retroarch.cfg file.
The easiest way to figure this out is to make a little drawing of your panel, label all of the switches as wired. Then, either run a test program or hook up the IPAC to a windows box and run WinIPAC. Then, mark on your drawing which buttons are sending which keys.
Finally, you should be able to look at a retroarch.cfg file which identifies the common gamepad button names, (a, b, x, y, etc) and you should be able to mark which IPAC keys should trigger these inputs based on which button/position you want to send each input.