Per the Incubator PMC's request, we have removed plugin.jar from source control and are no longer including it in our distribution artifacts. You will now need to manually ensure that this library is available on your classpath. See the following for more information.
Windows and Linux: http://java.sun.com/javase/6/webnotes/6u10/plugin2/liveconnect/ Mac OSX: http://developer.apple.com/qa/qa2004/qa1364.html
