On the PMDX-126, pressing the "test" button is supposed to light up the EStop LED, and send the EStop signal to Mach. That tells Mach not to try and do anything.
As for no power - if you have voltage at your power supply but not at the power terminals on your G320Xs (have you checked with a meter?), check your wiring. Does the motor power go through the large relay on the 126?