The MOUSEBUTTON() function returns whether the Mac's mouse button is pressed. It returns 1 if the button is pressed or 0 otherwise. It takes an optional parameter indicating the desired mouse button.
MOUSEBUTTON() === MOUSEBUTTON(1) === whether the left mouse button is pressed MOUSEBUTTON(2) === whether the right mouse button is pressed MOUSEBUTTON(3) === whether the middle mouse button is pressed
4, 5 and 6 may also be valid buttons depending on your mouse.