Hmmm.... an HP motherboard. Yeah, you probably won't be able to find a manual on that since it's part of a prebuilt computer. Pretty much your only option then is trial and error. You should be okay doing it, but sticking wires where they don't go is usually not good, so it's a risk. It's still unlikely that you would damage anything from guessing with those pins. Good luck!
EDIT> I'm assuming you've already removed the wires from the old case, but if not, you can just mark the pins as you remove the old wires.