When you address the Powermaster from the CAB2, and turn the Big Red Knob on the CAB2 to increase voltage, does an LED on the Powermaster blink - indicating CAB2 and Powermaster are communicating.
I do not know if this is true for CAB2 controlling Powermaster as it is for CAB2 controlling engine, but worth a look. The CAB2 has the slide control on the left - the Train Brake Slider. Make sure it is all the up to the top.