IPAC Ultimate just needs 5v for the LED drivers. It will run as a keyboard encoder with just the USB connected - so you only NEED the 5v if you have LED buttons/etc.
The 5v is just any red and black from the computer power supply. You can either cut the molex off the cable provided with the IPAC and splice it in to any red/black, or find an old female molex and splice that in first to the power supply - then you don't have to do anything to the original cable.