check if the name in jbpm-ds.xml matches one defined as data source in your persistence.xml, below is an example taken from jbpm-installer:
<?xml version="1.0" encoding="UTF-8"?>
<datasources>
<local-tx-datasource>
<jndi-name>jboss/datasources/jbpmDS</jndi-name>
<connection-url>jdbc:h2:tcp://localhost/runtime/jbpm-demo</connection-url>
<driver-class>org.h2.jdbcx.JdbcDataSource</driver-class>
<user-name>sa</user-name>
<password></password>
</local-tx-datasource>
</datasources>
jndi-name is that one that you should check
HTH