Your answer was in the docs, but you just missed it :)
Handling a mouse down event is handled by NSView's parent class NSResponder: https://developer.apple.com/library/mac/documentation/cocoa/reference/applicationkit/classes/NSResponder_Class/Reference/Reference.html#//apple_ref/occ/instm/NSResponder/mouseDown:
You just need to define mouseDown on your view.
class MyView < NSView
def mouseDown(event)
# what you want to do
end
end