I searched for similar questions on this issue but found none related to Mac OSX.
Situation:
Trying to test android apps on a Galaxy S (D
So i did some research because the same thing happened to me. If the simple solution doesn't work you should check if you installed knox when you first launch your phone. If you did install knox the only way for you to activate usb debugging would be to reset your phone back to factory config in order to deactivate knox (aka not installing it again), apparently you don't have the option to unistall knox on recent phones.