So if the DNS-320 is only showing up by IP ADDRESS in the Media Library Manager, that means that the WD could only see it via NFS.
And if only the P2P folder shows up, that would indicate that is the only share that is allowed to use NFS.
Sounds like there's an issue with the "Samba," or "Windows Sharing" configuration on the DNS-320 that's keeping the WDTV from discovering it.