I assume you want to play music, videos and view pictures stored on your NAS via your PS3? You need to enable the NAS's UPnP AV server, set the correct folder and save the settings.
The PS3 should then list the NAS in it's Photo, Music and Video sections of the XMB.
I have mine set up this way and it works great. The only oddity I have found (using FW 1.09) is that after adding new media I always have to refresh the AV server twice for the PS3 to be able to see the files.