<MediaElement x:Name="bgvideo" Width="800" Height="600"Source="/Videos/BG_LOOP_BIG.wmv" />

bgvideo.Source = new Uri(@"pack://application:,,,/Videos/BG_LOOP_BIG.wmv", UriKind.Absolute);

bgvideo.Source = new Uri(@"/Videos/BG_LOOP_BIG.wmv");

bgvideo.Source = new Uri(@"C:SomeFolderVideosBG_LOOP_BIG.wmv");

<MediaElement LoadedBehavior="Manual" x:Name="bgvideo" Width="800" Height="600" Source="Videos/BG_LOOP_BIG.wmv" />

bgvideo.Play()