Fortunato calls Montressor bad names and Montressor gets sick and tired of him always calling him names. This is the exposition because this shows the beginning of the story and how it leads to the rising action.
Montressor gets angry that Fortunato said those bad things to him and decides to get revenge. Montressor wants to get Fortunato drunk and lead him to the catacombs. This explains the rising action because it adds tension and drama.
Montressor gets Fortunato in the catacombs and has him where he wants him. Montressor chains Fortunato which traps him. This explains the climax because it's the most juicy part of the story.