Resolution:
You'll have to install Turbo XML to version assets in XML Canon.  From the web interface you can only upload new assets to the repository; you'll need a client application from the Turbo suite to create multiple versions of an asset.  Turbo XML is a separate client application that should be licensed and installed wherever an XML developer is required to version and edit XML files within XML Canon.  Through the client applications, users can version, check-in and check-out documents. Documents that are checked-out will be locked for editing and access by another user will be read-only until the document is checked-in.