Hi,
The D810 requires camera raw 8.6 which currently is not available for Photoshop elements.
You could download and install the free Adobe DNG converter and use that to convert the NEF files to DNG files which can then be processed by Photoshop elements.
MAC: http://www.adobe.com/support/downloads/thankyou.jsp?ftpID=5824&fileID=5839
Brian