accessing XMP data from a plugin

I am trying to access (and later modify) the XMP data that is associated with a document in Illustrator form a C++ plugin.

I thought that 

AIErr err = sAIXMLDocument->GetDocumentMetadata(&xmpRoot);

would give me the correct staring point, but it does not.

It returns me a node that only contains 2 entry points, 'variableSets' and 'sfw'

What is the correct starting point for XMP access/manipulation?

Frank