A few things can cause baked mac and cheese to be dry. For one, don't add more flour than the recipe calls for. Too much flour will over-thicken your sauce and dry out the pasta in the oven. In addition, be careful not to overbake the mac and cheese. Keep an eye on it as oven temperatures can vary, and don't bake it for longer than 30 minutes. If all else fails, up the amount of evaporated milk you use in this recipe for a thinner, wetter sauce.