Read MF's review. I haven't made this recipe but I would guess that you are NOT meant to add the ingredients on the box unless they include anything beyond what this recipe calls for. Eggs and fat are standard ingredients on cake mix boxes and you wouldn't want to double them, I shouldn't think. Hope this helps if you want to try. It's an easy mistake to make, so don't feel bad.
Question: I made this cake last night and it was horribly flat. Didn't rise at all. The directions don't mention it, but are you supposed to include the ingredients called for on the cake mix box as well?