When creating a new metadata in an existing workspace, the window for selecting the desired technology does not contain XML Schema (or any other expected technology).
When working with a new Workspace, this technology is available when creating metadata.
This can happen when working with a workspace which was created with an old version of Stambia where the technology would not exist yet.
Or maybe the workspace was corrupted.
The solution is to revert the Technology Definitions to their defaults. See this article.