Preheat oven to 375 degrees. Place macaroni in a 9x12 inch baking dish. In a medium saucepan, melt margarine over low heat. Stir in flour; cook 1 minute, stirring constantly until mixture is smooth and bubbly. Stir in milk; heat to a boil, stirring constantly. Add cheese and Rotel. Stir until cheese in melted. Pour over macaroni and stir to combine. Bake uncovered 30 minutes or until heated through.