The ML1000 is a bottom dweller. It doesn't support a CPT statement or indirect addressing. The number of interations is too small to bother with indexed addressing. You have to do it by brute force.
It shouldn't take more than five rungs.
ADD C5:0.ACC C5:1.ACC N7:0 (add the first two counters)
ADD N7:0 C5:2.ACC N7:0 (add the third counter to the total)
ADD N7:0 C5:3.ACC N7:0 (add the fourth counter to the total)
ADD N7:0 C5:4.ACC N7:0 (add the fifth counter to the total)