http://stackoverflow.com/questions/7386336/eclipse-xsd-editor
Apparently the XSD editor wasn't included in my package (Eclipse IDE for Java Developers). I added it using:
Help > Install New Software...
Work with:
Juno - http://download.eclipse.org/releases/Juno
Select:
Web, XML, Java EE and OSGi Enterprise Development
And then:
Eclipse XML Editors and Tools