Currently this functionality is not supported out of the box. RadMediaPlayer is a wrapper around Silverlight 3's MediaElement object and for the moment, slow motion and fast-forwarding (trick play) are not supported. Just for your information, this thread demonstrates how to achieve slow motion using the SmoothStreamingMediaElement object available in the IIS Smooth Streaming Player Development Kit 1.0 – Beta 1. Let me know if you have additional questions or comments.
the Telerik team