From everything that I read (for example, there or there ), I suspect that DGML is not available at all in the Professional version of Visual Studio 2010.
You will most likely need Visual Studio 2010 Ultimate. DGML seems to be part of the Visual Studio 2010 Ultimate Architecture features.
It also seems that DGML is available in beta version of Visual Studio 2010. (IIRC, beta was the release of Ultimate.)
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