The "pads" under the power and eject buttons usually don't have voltage...at least not in my model. There is a tiny voltage there, but the voltage reading from my fingers is much higher. These are capacitive touch sensors, so they have to work at very low voltage/current.
How to force the PSU to turn on:
One of the pins on the front of the PSU should detect as 0V, but when doing resistance to ground it should be relatively high. Apply a 3.3V voltage to this pin (us a linear voltage regulator), and the PSU will be forced to turn on even if there is no mainboard attached.