[Baertram]

I'd read this tutorial about GUI (XML):
https://wiki.esoui.com/SimpleNotebookTutorial/part6

You need to define a TopLevelControl and then anchor your buton on it.
Without the TLC you cannot draw any controls directly to the GUI.
Think about it as a rectangle where you anchor yoru button and other conrols to, like a menu, but not visible (unless you add a backdrop texture as background).

<GuiXml xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://sir.insidi.at/or/schema/esoui.xsd">
	<Controls>
            <TopLevelControl name="MyTestTLC" hidden="false" movable="true">
               <Dimensions x="200" y="200" />
              <Anchor point="CENTER" />
              <Controls>  
                 You button control etc. here
              </Controls>
           </TopLevelControl> 
	</Controls>
</GuiXml>

If you want to add the button to any existing control, you need to anchor it to that control so it shows (e.g. at the inventory).