I think that the climax is the night of the eight day, when they start to hear noises and when Madeline bursts into the room where he and Roderick are. The falling action is the ensuing struggle and when both Roderick and Madeline are dead. The resolution is when the narrator runs out of the house, and he then sees the house crumble.
However because of the way this story is structured, you could adjust it somewhat.
(Just to add to the answer above) The climax is when Roderick and the narrator realize that they buried Madeline alive and when she falls onto Roderick.