XQDT provides support for editing, execution, and debugging of XQuery modules in Eclipse.It requires DLTK version 3. Certain plugins will install DLTK version 4, e.g. Scala IDE, which prevents XQDT installation.
If that is not installed then version 3 of DLTK can be installed from the indigo update site
http://download.eclipse.org/releases/indigo
Then install XQDT