Thanks MartinL!
Adding the following line to the setup() function in the sketch in my post: http://forum.arduino/index.php?topic=339470.msg2422343#msg2422343 solved the problem of writing to the AC registers.
REG_PM_APBCMASK |= PM_APBCMASK_AC; // Set the AC bit in the APBCMASK register
Now I can use this piece of valuable information to develop interrupt triggers based on the two ACs in the M0 PRO/Zero.
Harry J. Whitlow