As far as I know, it's the same thing. Where did you get the recipe? It's the recipe I have, but mine calls for "dry skim milk" which is the same as nonfat dry milk, which is the same as "instant" milk.
There's another dry milk product that they may be referring to which is a whole milk dry powder called "NIDO"- this one has the fat content of 4% fat milk. Maybe that's the one they are specifying should not be used. :?