For I and J variables in G2/G3 in the PostBuilder, I found the "User Defined Expression" and now have just $mom_pos_arc_center(0) for I and $mom_pos_arc_center(1) for J.
This seems to put the value for the center point of the arc in I,J in the postprocessed file. I think this is what the MillPlus IT control is looking for according to the manual.
Too bad I don't have the machine here to try it out. Will be Monday before I can test this PostBuilder change.
Can anybody familiar with this control confirm what I am thinking?