Having difficulty installing Grado cartridge on a Rega turntable

I'm trying to install a new Grado Sonata 2 cartridge on my Rega RP3 turntable. I'm replacing the Grado Sonata 1 cartridge which was installed by the dealer who is now inaccessible. I understand Rega and Grado are somewhat incompatible. I'm having all sorts of problems installing the new cartridge and need help. It doesn't behave like the old one.
Does anyone know of a Rega service person in the Long Island, New York area?