Make sure you have proper definitions in your persistence xml, here is a sample from default installation (not complete just the relevant part, I think):
<persistence-unit name="org.jbpm.persistence.jpa" transaction-type="JTA">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<jta-data-source>java:jboss/datasources/jbpmDS</jta-data-source>
<mapping-file>META-INF/JBPMorm.xml</mapping-file>
<mapping-file>META-INF/ProcessInstanceInfo.hbm.xml</mapping-file>
<class>org.jbpm.persistence.processinstance.ProcessInstanceInfo</class>
<class>org.drools.persistence.info.SessionInfo</class>
<class>org.drools.persistence.info.WorkItemInfo</class>
<class>org.jbpm.process.audit.ProcessInstanceLog</class>
<class>org.jbpm.process.audit.NodeInstanceLog</class>
<class>org.jbpm.process.audit.VariableInstanceLog</class>
Important element is in bold, make sure you have it there and it is accessible as that file defines ProcessInstanceInfor entity. This is valid for hibernat 3.x usage (JPA1).
HTH