If you're 323 isn't listed under "video" "music" or "photo" (it should be under all three) your 323 or network is not setup correctly:
1.Make sure your router supports and has upnp enabled. Do an internet connection test on your PS3 (settings>network settings>internet connection test). It should succeed in every category. UPnP should be available.
2. On the dns-323 you must enable the upnp media server. Don't bother getting Twonky, or Mediatomb or any of the fancy ones at first: the built in one is fine, albeit very basic (advanced>upnp media server). Make sure the directory chosen here is the location of the media files you wish to share. Notice the refresh button - it is HIGHLY reccommended that you refresh the server any time you add new files, or change files names/file locations in this folder.
So check these things, and hopefully everything will be running smoothly.