So i would wait to see if anyone else agrees with this...I would connect Power(RED) and ACC(white) to the m2atx and then ground the green wire on the 20 pin harness. That is power good and tells the power supply that the motherboard wants or needs to turn on. Then you can check 5 and 12volts from the 4 pin Hard drive power connector. red is 5 and yellow is 12 in respect to the black ground pins. Make sure you are in DC and not AC on your multi-meter.
On standard computer power supplies you can plug the 120vac from your house and then short the green wire to ground to make the power supply turn on. I would think that the m2atx would do the same thing. I have one and would check but its burried in my carputer right now.