If it's a 320L NAS
Go to
http://dlink.vtverdohleb.org.ua/ (register for free if not already registered)
Click "Surveillance" top right
Then look top left for Surveillance Station - select DNS-320L version and it'll download.
Login to server - click Applications
Click Add-On Management
Click Choose File and browse to downloaded file and select
then click apply - this will install My Surveillance - if I remember correctly you need to set the addon to run