Here is what you need to do.
Find the directory for jdbc drivers in plugins directory (following is an example from http://community.jboss.org/people/bpmn2user/blog/2011/02/26/orcale-integration-with-jbpm5)
birt\ReportEngine\plugins\org.eclipse.birt.report.data.oda.jdbc_xxxx\drivers
Copy your jdbc driver into this directory.