Windows 7 (and i believe XP and Vista as well) does not allow you to index shares that exist on my Dlink NAS (DNS-323). The hard drives are formatted by the NAS with EXT-2 file system.
This prevents me from assigning a directory on the NAS to a library location.
There has been a previous submission of this error related to NAS shares, however, this one is more specific. Here is the referenced submission: http://www.windows7taskforce.com/view/1250
When it comes to music, I noticed a way to get a music directory part of your library.
If you use the Zune software to manage your music, if you specify a NAS directory as your main music location within the Zune software, that location shows up in your music library.
This, so far, is the only way i have been able to get a network location on the library from my DNS-323.