Preheat oven to 375'F. Cook macaroni as directed on package; drain and set aside. 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. Reduce heat and simmer until thickened, about 1 minute. Add cheese and Ro*Tel. Stir until cheese is melted, remove from heat. Stir macaroni into cheese sauce. Transfer to an ungreased 9x12" baking dish. Bake uncovered 30 minutes or until heated through.

From "Southwestern Sensations Starring Ro*Tel", courtesy Mike Orchekowski.