How do I edit Nikon NEF photo files in PSE 14?

See this thread:

Elements 14 camera raw plug in

If PSE 14 doesn't support your Nikon camera model you can use the Free Adobe DNG converter to convert the NEF files to DNG files that will load into your version of PSE.

Each new camera needs support in ACR. If you have a fairly new Nikon camera the version of ACR included with your older version of PSE doesn't include support for it.

Or upgrade to PSE 15.