In a large saucepan or skillet, cook the beef, onion, green pepper and celery over medium heat until meat is no longer pink; drain. Add tomatoes and Cajun seasoning. Cook, uncovered, for 15-20 minutes, stirring occasionally., Meanwhile, prepare macaroni and cheese according to package directions, using 2 tablespoons milk and 1 tablespoon butter. Stir into beef mixture; cook for 2-3 minutes or until heated through.