Meanwhile I actually made it work.
I didn't realize I actually need a Gtk widget not a Xwt, because that's what the addin is using.
I was able to add Mono.TextEditor.
But now I have trouble understanding some things regarding file editing works.
For example, how MonoDevelop.Ide.Gui.IViewContent:IsDirty or IsFile, IsReadOnly works
I want to make the "Save" option enabled in the File menu.
I guess it's something related to MonoDevelop.Ide.Gui.Document. I probably have to signal somehow that document is changed.
I've no idea how all this works.... I don't know where to look for this info