See https://github.com/krisv/jbpm/blob/master/jbpm-installer/db/persistence.xml, this is the persistence configuration where you can see it references a data source called jdbc/testDS1. In JBoss AS, you can configure such a data source by dropping a data source configuration in the deploy folder, like this one: https://github.com/krisv/jbpm/blob/master/jbpm-installer/db/testDS1-ds.xml
In tomcat, this is slightly different. Refer to http://tomcat.apache.org/tomcat-6.0-doc/jndi-resources-howto.html for JNDI configuration on Tomcat6.
Kris