Heat oven to 350°F. Line each taco shell with 1 slice of cheese. Arrange taco shells on ungreased cookie sheet (cheese will fall to one side). Bake 5 to 7 minutes or until cheese is melted.
Meanwhile, in 10-inch skillet, cook beef over medium-high heat 5 to 7 minutes, stirring frequently, until thoroughly cooked; drain. Stir in dip. Cook until hot.
To serve, spoon about 2 tablespoons beef mixture into each warm taco shell. Top each with lettuce and tomato. Serve with salsa.