The Xonar DX sound card gets its power from the PCI slot it's plugged into. a separate power cable is not needed or required. The only connections on the card are the outputs for your amplifier, mic & line inputs and the digital S/PDIF.
OK...Just looked in the correct manual... The DX DOES require an additional power cable to work..My bad
from the pictures it looks like either a standard power cable for a 3.5 " floppy drive, or Asus should have supplied an adapter that would take their power connector and matched it up with a standard molex plug from your PSU