I figured out the solution.
Copy the following and paste it into <jboss-as>\server\default\deploy\jbpm-gwt-console-server.war\WEB-INF\classes\META-INF\ CustomWorkItemHandlers.conf
[
"Log": new org.jbpm.process.instance.impl.demo.SystemOutWorkItemHandler(),
"Service Task": new org.jbpm.bpmn2.handler.ServiceTaskHandler(),
]
Restart jboss after making above changes.
This works