Where is the Additional Medicare Tax from Form 8959 accounted for in my TurboTax return? I don’t see a place to add it and don’t see it reflected in my summary.

It will show up on Sch 4 of the Form 1040 and then ends up on line 14 of the Form 1040.

You do not actually add it, TurboTax will do this based on income on the return.