There are two advanced tools in Schema Workbench that enable you to work with raw MDX and XML. The first is the MDX query editor, which can query your logical data model in real time. To open this view, go to the File menu, select New, then click MDX Query.
The second is XML viewing mode, which you can get to by clicking the rightmost icon (the pencil) in the toolbar. This replaces the name/value fields with the resultant XML for each selected element. To see the entire schema, select the top-level schema entry in the element list on the left of the Schema Workbench interface. Unfortunately you won't be able to edit the XML in this view; if you want to edit it by hand, you'll have to open the schema in an XML-aware text editor.