OpenMeetings default configuration is to use Apache Derby.

It is recommended for production environments and high availibility to change to an usual relational database like MySQL, Postgres or DB2.

For more information about Apache Derby see http://db.apache.org/derby/

There is a sample configuration for Apache Derby that ships with every release in:
/webapps/openmeetings/WEB-INF/classes/META-INF/derby_persistence.xml

If you encounter issues, you can drop the db and then run the web based installer again