Sometimes the Purser is the one greeting you at the entrance to the plane.. sometimes s/he is somewhere else, though probably not in the back of the plane. If you don't know, then just ask someone.

To clarify my earlier post, "enough" is at least 15 pieces of chocolate. Stick to one box (not multiple boxes). I think it's best to deliver it as an afterthought - seems less contrived that way. Personally, I would think you want to avoid the idea of looking like you are trying to bribe the Purser.