LR 5.6 is needed, and your destination needs to be writable by LR, and you cannot have touched the files with Nikon software that is old, like what you might have used with your previous Nikon camera, to transfer or view the images before importing into LR.
Obsolete Nikon software changes something in the metadata of newer-camera NEFs that makes the raw files unrecognizable to Adobe software. You can correct the corruption with a utility from here:
http://owl.phy.queensu.ca/~phil/exiftool/fix_corrupted_nef.html