Since kale is essentially just lettuce or cabbage, I would assume it will be used like lettuce on top of burgers, in side salads and crumbled into other things. According to news, the kale will be part of breakfast bowls.
I'm fine with adding kale to stuff but I feel adding it to the food at McDonald's is basically the equivalent to ordering everything on the left side of the menu and then washing it down with a Diet Coke.