As explained by Franz , the solution is to use idMso. For the New Slide team you are looking for, if you look at the MSN in the idMso Table for the New Slide, you will find two entries. The one you are looking for is a Gallery with idMso = SlideNewGallery. (not a button). You can add it to XML. I like to use the Ribbon Editor . With the feed editor, it looks like this: 
And in the add-in, it looks like this: 
The corresponding part of the CustomUI XML component is as follows
<group id="TD_GrpMisc" label="Misc"> <gallery idMso="SlideNewGallery" size="large"/> <button idMso="SlideNew" size="large"/> </group >
Thierry dalon
source share