Windows 8 will strictly stream to only DLNA approved devices. Grace Digital Internet radios are not yet DLNA approved, so this won’t be possible direct from Windows media player.
To get around this, you can set up another media server such as Twonky, TVersity, MediaMonkey. If you do a google search for UPNP media servers, you should be able to find one that you like. They vary in price and capability.