Yes schema autocomplete is supported very well, what example XML file were you having trouble with? It’s most likely your header is wrong.
Create a new web project with version J2EE 1.4, then open the web.xml file, it uses a schema for validation and autocomplete, you can use that as an example of how a XML file using a schema should look. (or it’s possible the schema you are referencing cannot be accessed/downloaded by the editor if you are behind a proxy).