If you are interested in adding equations to your XML documents, you need to make sure that the "MathML support" add-on has been installed. This is done using menu item " → ".
This MathML add-on:
allows to create standalone MathML documents,
tightly integrates MathML presentation markup with DocBook 5,
tightly integrates MathML presentation markup with DITA topics,
tightly integrates MathML presentation markup with XHTML 5.
How, in practice, to add equations to your XML documents is described in How to add equations to your XML documents. The precise contents of the add-on is detailed in Appendix B, Contents of the "MathML support" add-on.
Native support means that MathML elements are treated like any other element. That is, XMLmind XML Editor does not embed a specialized math editor. On the contrary, all commands, all kinds of selection, work fine with MathML elements. However, using the generic commands requires you to learn MathML. If you don't want to do that, for example, because you just want to type a couple of equations, then simply use the MathML tool which is installed by the add-on just below the Edit tool, at the top/right of the main window. Using the MathML tool is explained in Section 3, “Using the MathML tool”.
 The content markup is not supported at all. Content elements are rendered as tree views.