More wiring, less worrying.
It will work. Or it won't.
Once wired and hooked up, try a game with mame.
If it all works, you are done.
If it doesn't work at all check that it is plugged in.
If this doesn't fix it, check your wiring (preferably with a multimeter on its continuity setting).
If it doesn't work, open notepad, mash some buttons and move the joystick to see if anything at all happens. Look at the keymap for the Ipac to see the expected behavior (eg. joystick moves curser, button X makes letter ? appear on screen, etc).
If it partially works, check the wiring for the non-working buttons.
If you can't figure it out, post here with a description of symptoms as best you can.
More building, less worrying. It's low voltage stuff and hard to mess up catastrophically.