IRibbonExtensibility needs to be ComVisible

When developing an Outlook add-in using VSTO 4, if you are implementing the IRibbonExtensibility interface (for customizing a Ribbon, for example) you might encounter cases where your Ribbon or menu items don't appear in Outlook's UI.
In those cases, you should check the following things first:

1. Your XML is valid - invalid Ribbon XML, such as placing the node inside the node are common mistakes, which will prevent your controls from appearing
2. Your class should be marks as ComVisible:
   [ComVisible(true)]
   public class Ribbon : IRibbonExtensibility
   {
      ...
   }