Prepare the mac'n cheese according to package directions. (I have also made this using the 7.25 oz. box of Kraft's Thick & Creamy Macaroni & Cheese, and it tastes just as good, or even better, but you will need to add a 1/2 cup of milk as stated on the box.)
Just before serving (and while still hot), stir in the salsa.
*Note: Whenever I make this recipe adding salsa that's has more heat to it (like my husbands homemade 'hot' salsa), I only add 4 tablespoons to the mac 'n cheese, but if I use a store bought brand that's not so hot, and is a medium-heat tasting salsa, then I'll add 5 tablespoons of the salsa.