Being unfamiliar with UPnP, I thought that I would post this because I got ZERO help from e-mail tech support and from my phone call with tech support.
Log in to the DNS-323, go to the advanced tab and the UpNp server tab.  Then, enable it and browse it to your music folder.  Save settings.
Now here is what I did NOT do and it did not work until I did a "refresh".  Once I did this, then I could power up my DSM-510, go to server and select the DNS-323 and it would actually find music.
The media server program is Garbage.  I used it for a while, and it was problematic.  So if you have both a 323 and a 510, you can set it up without running a PC.
Next, I need to figure out why the 510 does not play .wav files.