Try moving your driver JAR to directory [jbpm.install.dir]/lib/mysql (or any location under the lib directory). That is the location included in the classpath of build.deploy.xml:

<path id="classpath.ant">
 <pathelement path="build/classes.identity" />
 <pathelement path="build/classes.jbpm" />
 <pathelement path="build/classes.webapp" />
 <pathelement path="src/config.files" />
 <!-- includes all jars under the lib directory -->
 <fileset dir="lib" includes="**/*.jar" />
 </path>