I am not a RibbonX expert but only a dabbler. The person who answers questions about the Ribbon the most is Greg Maxey. He does respond to questions here and may see yours. In the meantime, here is a link to his lead page on Ribbon modification:
Customize the Ribbon (It doesn't take rocket science)

The book that I use is RibbonX: Customizing the Office 2007 Ribbon - by Robert Martin, Ken Puls, and Theresa Hennig and that is where I would be looking for an answer.

I am assuming that you are working directly with the XML and not using the ribbon modification in the Word user interface for your work.