In recent versions of the free and open source Eclipse IDE you can generate XML documents from DTD and XSD files. Right-click on a given *.dtd or *.xsd file and select "Generate -> XML File...". You can choose which root element to generate and whether optional attributes and elements should be generated.
Of course you can use Eclipse to create and edit your DTD and XSD schema files, too. And you don't need to install any plugins. It is included in the standard distribution.