One possible reason your Xbox 360 can't see the NAS is that it's not DLNA capable. The PS3 is and there are some known issues that cause the DLNA errors.
http://www.dlna.org/about_us/about/If you go to the DLNA website the Xbox 360 (or any Microsoft) product is not in their list of items that are DLNA capable.
If there are people who's Xbox 360 does see the NAS and can access it then I'm at a loss but then I admit I'm not an expert on this. This is just info that I've found on my own.
Check the other threads as I know there has been previous history on issues of this nature concerning the gaming consoles and the DNS-323.
I use my PS3 with the DNS-323 for over a year now and aside from the occassional DLNA errors (to which I usually can correct with a UPnP refresh, shut down and back on) I have been running fine.
Hope this helps in some way. Good luck.