How is IXRTextBlockPtr used?

I have a XAML file that is made by Expression Blend. But the Silverlight project that corresponds to it has to be in C++ instead of C# (it has something to do with being part of an embedded system).

The automatically gernerated C++ code referrs to the TextBlock code in the header file like this:

    IXRTextBlockPtr            m_pFoo;            // <TextBlock x:Name="Foo">
So how do I assign a value to the text block such that it appears in the window defined in the XAML?