Older versions of the Nikon software have been known to make the images unreadable by Adobe software. There is a utility that will fix them, but I don't have it. Perhaps someone else will be able to provide that for you. However, are you sure the DNG converter isn't recognizing your NEF files? It is prompting you for the FOLDER. You should choose the folder containing your files, but don't open it. That is a problem that is often encountered when first trying to use the DNG converter.
