If you are trying to use WMP11 as a server you must first allow the DSM520 into the software. This link will show you how to do that
http://support.dlink.com/faq/view.asp?prod_id=2692&question=DSM-120%20/%20DSM-320%20/%20DSM-320RD%20/%20DSM-510%20/%20DSM-520Once you have done that WMP11 will serve media to your DSM520 if they are on the same network.
When you select My Media from the home menu on the DSM520 a list of media servers should be listed. I have noticed the message "No Servers Found" when I first go to this screen but after a few seconds my media servers finally show up in the list. If you are using your PC as the server the name of your PC will be in the list.
If you have allowed the DSM520 into the WMP11 software and still have the "No Server Found" problem then I would look at the network setup and make sure the DSM520 is connected to the network. Make sure the DSM520 is connected to the same network as the PC.
Is there a firewall in your router or PC that is blocking the DSM520? You should be able to allow the DSM520's IP address to pass through the firewall.
I hope this info helps.