@Ace it looks like you didn't add it correctly, did you make an update_NFUL function in powder.h? i can't tell much else from just that, maybe upload the source real fast.
is nful.c being included in the project, like does it show up inside visual studio menu thing on the left. you should be able to add an existing file into the project fairly easy. EDIT: the same for elements.h, check to make sure it is in the headers part of the project. also, have you successfully added any other elements.
EDIT2: on a related note, the code you have there wouldn't work anyway, you are missing some brackets for the for loops, it is not including the if inside it currently.