There are two boxes to enter the codes, on Screen13.1.  Put the 1 in the first one, put the M in the second one - then enter the amounts (total distribution & taxable amount).

Or is there more to the story?

Code M is for 'Qualified plan loan offset'  -  does 'rollover' apply here? 

I just had one of these too. It's taxable up to the amount showing on the 1099R. Rollover does not apply. It was a reduction in the plan balance to pay a loan that was in default.