Benoit and Schmoll in “Adding Machine: A Musical” There’s a cliché out there that an opening number can make or break a show. Unfortunately, the first song in Adding Machine: A Musical, now being staged by Studio Theatre, contributes to the “break” side of the equation. Though the first scene’s staging, with its central couple Mr. and Mrs. Zero strapped to an upright mattress, as if we’re peering down into their bedroom, is pleasantly…