if the 5v line on the IPAC isn't used, you can use that to light the LED. Otherwise, you can tap into a molex connector from your PC power supply.
As far as parts... all you need is the LED and the correct resistor for whatever voltage you'll be using.
Edit: You could also just get one of those LED cannons that work with a USB plug and just use that. It'll be a few bucks more than if you do it yourself, but you can install it in 20 seconds.