Gazillionaire David Koch, who led the funding for American Ballet Theatre’s new “The Nutcracker” at the Brooklyn Academy of Music, started off his curtain speech by putting his foot in his mouth: “Thank you, Peter, uh, Kevin,” apparently mistaking ar tistic director Kevin McKenzie for rival New York City Ballet artistic director Peter Martins, and sparking gasps from the au dience. The screw-up was un derstandable. Koch has given millions to both companies, but he’s a better industrialist than public speaker.