Telerik MediaPlayer is able to play media from stream. You could check following help article about smooth streaming: http://www.telerik.com/help/silverlight/radmediaplayer-smoothstreaming.html and check our online demo: http://demos.telerik.com/silverlight/#MediaPlayer/SmoothStreaming (an example how to play media from stream).

You could take a look at the following MSDN article about the UDP support in SIlverlight: http://blogs.msdn.com/b/ncl/archive/2009/11/18/udp-multicast-in-silverlight-4.aspx.

Finally, I'm not sure why you are getting the MediaElement of RadMediaPlayer to be null. There are several scenarios that cross my mind:
1) If you have changed the control template, there could be no MediaElement in the template.
2) If you are trying to retrieve the element before the control has been properly loaded. The Loaded event would be a good place to retrieve the MediaElement.
3) If RadMediaPlayer is created programmatically and has not been added to the visual tree.
If your case does not belong to neither of the above scenarios, the best way to assist you would be to provide (a download link)  us a simple project that we could investigate in detail.

Hope this helps. Don't hesitate to contact us if you need further assistance.