VSTO Excel Com Add In Ribbon Not Loading

I encountered the same problem. I had an incorrect path to the XML file and it was returning empty:

 public string GetCustomUI(string ribbonID)
 {
    return GetResourceText("CheckThisIsTheCorrectNameSpace.Ribbon.xml");
 }

Hard coding a Namespace as a string is not a good idea, mainly because they are not refactor-able as well as the helpful comments below.

You should remove your override of CreateRibbonExtensibilityObject. By default, this is implemented by ThisAddIn's base class, and calls CreateRibbonObjects. You can either override CreateRibbonObjects (this should return an array of all of your Ribbon objects), or just let the default implementation of CreateRibbonObjects do its thing (which is to say, reflect over the entire assembly everytime that your addin starts up).

You can read more about how all of this ties together in this blog post

I have the same problem when trying to change the default TabAddIns. What i see is that idMso is for office bar tabs and id is for new tabs. The following works for me.

<tab id="TabAdvComTracking" tag="TabAdvComTracking" label="Adv.com Tracking" visible="true" insertAfterMso="TabAddIns">