you should have these mapping files defined for JPA2 in your persistence.xml file:
<mapping-file>META-INF/JBPMorm-JPA2.xml</mapping-file>
<mapping-file>META-INF/ProcessInstanceInfoMapping-JPA2.xml</mapping-file>
you can take a look at one included in jbpm-gwt-console-server.war as it uses JPA2 by default since jbpm 5.4.
HTH