Arrange carrot in a vegetable steamer over boiling water. Cover and steam 6 to 7 minutes or until crisp-tender. Set aside, and keep warm.
Combine flour and 2 tablespoons skim milk, stirring until smooth. Combine flour mixture, remaining 1/4 cup plus 2 tablespoons milk, milk powder, and margarine in a small saucepan, stirring well. Cook over medium heat, stirring constantly, until mixture is thickened and bubbly. Remove from heat; stir in horseradish, mustard, and salt. Combine carrot and horseradish mixture; toss gently to coat.