Add button to existing ribbon panel

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I have followed this post by Jeremy Tammik on how to add a button to an existing ribbon panel, but I am facing the same problem: the button is added but it's disabled. Not much use for a button when you can't click on it!
http://thebuildingcoder.typepad.com/blog/2013/02/adding-a-button-to-existing-ribbon-panel.html
Right now, I am creating my own ribbon, adding panels to it and adding two buttons. But I don't want to create a whole new ribbon for just two buttons. I would like to add my buttons to the "Architecture" ribbon (which is called "build_rac" internally). The way I have implemented my Addin right now it's using the "commandData As Autodesk.Revit.UI.ExternalCommandData" to communicate with Revit. With the example by Jeremy, there's no "commandData As Autodesk.Revit.UI.ExternalCommandData", so even if I do get my button enabled, I can't interact with Revit.
I am stunned why accessing the ribbon is this difficult!!!
Any help is appreciated!