I have another way to solve this issue.
Although there is a intall jbpm human tak war task (the example used in Chapter 3) in build file, but this war file doesn't run on my JBoss 7 server.
So I deploy $jbpm\jboss-as-7.0.2.Final\standalone\deployments\jbpm-human-task-war.war via WEB GUI.
Done!