In vanilla processing mousePressed is both a variable and a method, for propane the boolean variable is available as mouse_pressed?, whereas you can create events/operations that occur on mouse pressed by adding a [mouse_pressed][mouse_pressed] method as below:-

def mouse_pressed  
  # code gets executed on mouse pressed
end

For sketch with mouse_pressed method see drawolver, follow link for a sketch using mouse_pressed?.

See also:-

mouse_released, mouse_dragged, mouse_clicked