Bring this to radio shack and show/tell them what you are trying to do:
You need to make two of these:

Label each one, #1 and #2.
Once you have those made, the left side connector in the above pic goes into one of these:

This end will goto the motherboard.
Now you should have 4 wires, take one from #1 and one from #2 and push them into one of these again:

This will goto the old PWR SW that was connected to the motherboard.
Do the same with the other #1 and #2 wires and connect this to your push button.
Z