AT90USBxxxx, LUFA driver for Mac OS x?
It depends which device class you implement. If it is a keyboard or mouse (HID class) it will just appear to the Mac as a normal keyboard or mouse would do. If it is a mass storage device ("USB drive") it will be a drive etc.

You decide based on you needs what you want to implement. If you need a custom device class you will also need custom drivers. Look at LUFA demos to see typical device examples.