Key extraction is a different story besides the viewing part in WhatsApp Viewer. There is an official guide posted on the XDA Forums regarding the key extraction procedure. Access the website and use the information on the page to safely extract the key from your phone. Follow the instructions properly or the process will fail.
Secondly, use the updated version of the software and attach the key file as well to decrypt the messages. You'll find it on the Software Informer application page.