Hello Kimi,
I had to update hibernate.cfg.xml, persistence.xml and testDS1-ds.xml in jbpm-installer\db folder, and copy mysql-connector-java-5.1.6-bin.jar to db/driver folder, you need to run ant install.demo again for these changes to take effect.
I had also to open jbpm-human-task-5.1-SNAPSHOT.jar located in runtime folder and change hibernate settings in persistence.xml
Greetings,