Is it possible that you need to register the taskhandler by setting up and editing a session.template file?
See items 7&8 at https://community.jboss.org/message/807163#807163 which in turn link to section 12.1.4 of the documentation (http://docs.jboss.org/jbpm/v5.4/userguide/ch.console.html#d0e4698).
These discussions are about WorkItemHandlers but as there is a section in the session.template file for eventListeners it may be that you need a similar solution.
Regards
Andy McC