how to add System.Windows.Controls.Frame to my reference in my project

The following code runs on SL4.. this works:

((System.Windows.Controls.Frame)(this.Parent)).开发者_Go百科Navigate(new Uri("/Import",UriKind.Relative));

but I want to add the reference System.Windows.Controls.Frame to my project and I cannot find it. why is that?

(I right click on my proj > Add Reference... on .NET tab)

There is no System.Windows.Controls.Frame assembly... The Frame class is in the System.Windows.Controls.Navigation assembly