I am not sure I fully understand your setup, but it is only logical that your NAD can play files it has access to on a computer or on a network.
Else it wouldn't be a network player.
That it can output to a USB DAC that is connected to a Windows PC is probably the result of Windows' UPnP feature.
But if I am correct you are asking for the opposite here.
You want the NAD to play what MusicBee tells it to.
So the NAD would be the 'slave' then.
For that MusicBee would need to speak UPnP/DLNA language to the NAD and the NAD should understand it.
But it doesn't.
(which is a bit odd to be honest for a device in that price range)
But then again, the DLNA plugin that is available for MusicBee is not bug free and is certainly not working flawlessly for everybody.
So there wouldn't have been any guarantee that it would have worked for you anyway.
Another option would be to use the NAD as a DAC and connect a computer containing MusicBee to it by means of a USB cable.
But, that feature also seems missing from it looking at the specs and the manual.
What I don't understand though is that they do have a USB audio driver download available.
Which suggests using it as a DAC would be possible.
You could contact their support to find out what's what, and if they are maybe planning a firmware update that supports UPnP/DLNA in the future.
P.S.
You may want to change the title of the post, since it seems your question is actually about:
'Control network player with MusicBee'