Take a loot at jboss-4.0.x/docs/examples/jca/mysql-ds.xml. The extra elements you see in the provided jbpm-bpel-ds.xml are for setting up an in-process hsql database. Since your mysql database runs in its own process, you don't need to set it up.
Make sure the hibernate.connection.datasource property in your hibernate.cfg.xml matches the jndi-name of your datasource. Remember that the jndi-name is prefixed with "java:".