The database file can be found here: C:\Documents and Settings\<username>\test.db if you are on windows. It is not in the jbpm-installer directory. You possibly could repair a broken db with the from h2 provided tools:
http://www.h2database.com/html/download.html
I have been able to backup the db and restore it with the tool found under: http://localhost:8082 (after installing the above).