launch the supercollider application and type MaxPat. Double click it and then press cmd+Y. You should see that the class file pops up. That means that your class was correctly loaded so it is ‘recognized’ by the program. If not, please check that your extensions folder is on the correct search path. You can also place the class on the main sc folder, on the SCClassLibrary.
If you still have problems, please copy+paste here what the post window yields so I can take a look.