You need Lightroom 5.3 (or later)
Or, you can download the free DNG converter (version 8.3 or later) and use that to convert your NEFs to DNG, and those will import into Lightroom
You need Lightroom 5.3 or newer in order to work with the NEF files. You can convert your files to DNG and use them with what you have now. Your choice. In order to have Lightroom 5.3 (might as well go to 5.7) you will have to purchase an upgrade.