How do I load Page with animation in wpf

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WPF

Hai
I want to load page with animation
I use this code

<window.resources>
<!-- Slide and Fade -->
  <storyboard x:key="SlideAndFadeIn" xmlns:x="#unknown">
     <thicknessanimation duration="0:0:.75"
                                Storyboard.TargetProperty="Margin"
                                From="500,0,-500,0"
                                To="0"
                                DecelerationRatio=".9" />
     <doubleanimation duration="0:0:.25"
                             Storyboard.TargetProperty="Opacity"
                             From="0"
                             To="1" />
  </storyboard>
</window.resources>

<Frame x:Name="Mframe" Content="Frame" Grid.Row="1" NavigationUIVisibility="Hidden"  Navigating="Mframe_Navigating" HorizontalAlignment="Right" Height="799" VerticalAlignment="Bottom" Width="1432" RenderTransformOrigin="0.5,0.5">
            <Frame.RenderTransform>
                <transformgroup>

                </transformgroup>
            </Frame.RenderTransform>

            <Frame.Background>
                <lineargradientbrush endpoint="0.5,1" startpoint="0.5,0">
                    <gradientstop color="Black" offset="0" />
                    <gradientstop color="White" offset="1" />
                </lineargradientbrush>
            </Frame.Background>
        </Frame>

And in Mainform

Private Sub RibbonMenuItem_Click(sender As Object, e As RoutedEventArgs)
  Mframe.Content = New PageReport()
  'Mframe.Source = New Uri("CupPage.xaml", UriKind.RelativeOrAbsolute)
End Sub

Private Sub RibbonButton_Click(sender As Object, e As RoutedEventArgs)
  Mframe.Content = New CupPage()
End Sub

Private Sub Mframe_Navigating(sender As Object, e As NavigatingCancelEventArgs) Handles Mframe.Navigating
  Dim s As Storyboard = CType(TryFindResource("Mframe"), Storyboard)
  s.Begin()
End Sub

this shows error (s.Begin())
please tell me whats wrong or Pls Tell me how to load a page with animation.

thanks