Simply install the plugin from the official plugin site:
and your are done. For a more detailed explanation on how to use XMind, have a lookt at the help section on the vendor's homepage.
The plugin is available under two licenses: the Eclipse Public License v1.0 (EPL)
and the GNU Lesser General Public License v3 (LGPL).
If you are wondering, how and why such a great tool was released as Open Source, the solution to this is simple. You can buy a XMind Pro version, too.
This contains additional features. Among them are additional export filter for MindJet MindManager, Microsoft Word, Microsoft PowerPoint, PDF, RTF.