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You need to change your build.xml file..

<java classname="org.jbpm.DemoTaskService" fork="true"> 

        <sysproperty key="jbpm.user.group.mapping" value="..........jbpm-installer/jboss-as-7.1.1.Final/standalone/configuration/roles.properties"/> 

      <classpath>

            <pathelement path="${install.home}/task-service/target"/>

            <path refid="classpath.human.task" />

      </classpath>

    </java>

Add the line in bold..