I would have to say the manual is the best source of information on the Circle Mill event.
A couple not-so-obvious facts:
Since the center of the circle mill is described in the event, it is not necessary to position the tool to the center of the pocket. Just be aware that the tool will return to the original position after the circle mill is complete.
You must activate a tool diameter in an M-Function event prior to the Circle Mill event, so that the control knows the diameter of the tool being used. This is necessary when pocketing to determine the proper overlap between passes.
Although a more recent manual (like a Delta 40/50/60 mill manual) can be used, be aware some variables were added, like starting angle (A, I think).