Mendix widgets can be used in the same way as Dojo widgets. Actually, Mendix widgets are Dojo widgets and so they can be instantiated via markup with a dojoType. MxButton for example has dojoType "mendix.widget.MxButton", and can be instantiated as follows:
<div dojoType="mendix.widget.MxButton" title="Title" ... ></div>
To see how existing Mendix widgets are used, check the exported forms (mxf-files) in your deployment/web/forms directory.