Available for a limited time, the new menu item is available either a-la-cart or as a Happy Meal entree in at least one location in Cleveland, OH.
The price for McDonald's Mac & Cheese alone is $1.75.
Mac & cheese is one of those comfort foods that haven't made much of a dent on fast food menus (possibly because it's fairly easy to make the boxed variety at home). Burger King tried offering it a while back as a "healthier" option that has since been discontinued.
I would guess McDonald's version would be the same add hot water type of deal as their oatmeal but I haven't been able to find any more details to confirm that.