More than a decade after they first surfaced to critical shudders and head-scratching, the unhappy have-nots of “Assassins” — the glitteringly dark musical by Stephen Sondheim and John Weidman about Americans who dream of killing their country’s presidents — have finally made it to Broadway…The frightening title characters of “Assassins” are restating their demand to be noticed in the Roundabout Theater Company production, which opened last night at Studio 54. And under Joe Mantello’s direction, they are doing so with an eloquence and an intensity that makes a compelling case for a misunderstood show.”