Build Plugin

Hi,

I am a beginner from openfire. I am trying to develop first plugin “hello world”. I follow instruction with this document http://community.igniterealtime.org/docs/DOC-1200

but when I try to build my plugin from eclipse I receive follow errors:

Buildfile: /Users/Stefano/Documents/workspace/Openfire XMPP Server/build.xml

** [taskdef] Could not load definitions from resource net/sf/antcontrib/antcontrib.properties. It could not be found.**

BUILD FAILED

/Users/Stefano/Documents/workspace/Openfire XMPP Server/build.xml:35: taskdef class com.oopsconsultancy.xmltask.ant.XmlTask cannot be found

** using the classloader AntClassLoader[]**

it happen if I try to build all plugins or single plugin. Someone can help me ?

thanks