Well, obviously you'd need 2 wires for powering the lights. I got the small illuminated PBs (don't be fooled, small means about 1 inch wide, not like Missile Command buttons!) I'm planning to convert mine to use LEDs. Yes, I know they sell LEDs for this on the Happs site, but I've got a bunch of LEDs lying around.
There are two extra metal tabs on the sides of the buttons, which take .250 quick disconnects (I think...) I'll be making a simple board for powering the LEDs. It's just a matter of soldering them in parallel, then finding the right resistor to use. Or you could just use the bulbs that come installed, and use +12V from the PC. Also, if you order Happ's LED replacements, I believe they come with a resistor already installed.
The power for mine will be coming straight from the PC via a hacked USB cable (+5V). You can just daisy-chain the grounds and the +5V inputs, but don't get them mixed up with your Ipac wiring! It'd be wise to use different colored wiring for the lights just in case... I should be putting this together soon, so I'll post a schematic of the wiring involved.