Identifying both sets of wires is pretty easy without resorting to tracing them all the way back to the motherboard.
1. Turn system on. Usually there's a power button on the mobo itself, or just put a piece of metal across the two pins for the power switch until it powers on.
2. Plug the wires labeled PWR LED - and PWR LED + into the mobo.
3. Plug one set of wires onto the + and - pins for the LED in the switch.
4. If no light, swap terminals (reverse that set of wires from + to - and - to +).
5. If no light, those are your power wires. Put them on the NC and NO pins. Polarity not needed. Either way will do.
6. Grab the other set of wires. Plug the set of wires onto the + and - pins for the LED in the switch.
7. If no light, swap terminals (reverse that set of wires from + to - and - to +).
8. There should be light now.
There's a 1:4 chance of getting it correct the first time. If you get all the way to step 8, you've just hit the longest odds. You should probably go to the horse track while you're so lucky. There will be no problem if you do it incorrectly. Nothing will blow up. The LED will simply fail to light up if wired backwards. It is a diode. Diodes, and LEDs (Light Emitting Diode) pass current only one direction. They act like a one way valve in a pipe.