As I understand your question :
Open your VS2010.
Then click the first button in opened toolbar whose hint is : "Create a new graph document from all ..."
Right click on newly opened document and click on
You will see the DGML.
Add this line:
<Link Source="a" Target="b"></Link>
section. and go to Graph1.dgml wich is updated with two new nodes.
For other option in dgml use google to find more.
Hope it helps.