You're probably fixed by now but for DB insertion:
Dig round in the bugfix version on Google (*not* the original code). Read the lower posts here,
https://coderanch.com/t/622325/jforum/Preserving-customizations-upgrading#2845374
Goto the appropriate subfolder of /config/database for your database. There you will find scripts which pre-populate (and can tear down) JForum databases e.g. for MySql try,
/jforum-2.3.4/WEB-INF/config/database/mysql
the
mysql_db_struct.sql script builds tables, we're not interested in that. But the
mysql_db_data_dup.sql script populates tables, so we are interested (!). Here's the script used to create the demo category/forum,
Should be roughly the same form for the original JForum code.
If you tried something similar, then the problem is not there.. This script code suggests that, so long as there is an initial setup defining roles and the like, it's that simple - but I would still heed the warning in the post above.