Head over to
SmashBurger on Facebook and "like" them. Then click on the "Fresh Mex Coupon" tab towards the top of the page. Fill out the requested information and you will get the coupon in the email address you provide.
The coupon is good for "Free SmashBurger entree with the purchase of a Fresh Mex Burger or Chicken Sandwich."
The coupon expires 04/08/12.