You can find my team's MSDN site here and download the first technical preview of our toolset here. In this download, you'll find the object model editor (or metamodel editor, or domain model editor, or Corona prefixed to any of the preceding - we're not what you'd call settled on naming just now). This tool is the first of our editing experiences for creating a domain-specific designer. (I posted a snapshot in this entry - it’s the first picture). With it, you can build the domain-specific metamodel that is the foundation for a visual language designer. Once the notation and the remainder of the designer experience are added, instances of this model are what your visual language designer will create.
Don't miss the walkthrough document which takes you through building a sample model with the tool. We're working hard now to get the remainder of the bits into your hands so you can try integrating designers into Visual Studio yourselves.