Assuming you're running JRiverMC on a PC (not on your NAS), then you need to connect the PSAudio to the PC via USB, not to the NAS.
If the PSAudio has a 'network bridge', you could also connect it to your router or network switch and set it up using DLNA.
If you're in fact running MC directly on the NAS, this is probably not supported either by JRiver or by PSAudio.