You will need to provide a database for your b2evolution installation.

This assumes you have some knowledge of MySQL, and already have it
installed and configured. If not, please refer to the Gentoo MySQL
guide at the following URL:

http://www.gentoo.org/doc/en/mysql-howto.xml

Once you have a database ready all you need to do is to go to this 
location

http://${VHOST_HOSTNAME}/${VHOST_APPDIR}/install

and provide the credentials required for the database access.

Note the administrator password b2evolution will provide and after
logging in with these credentials you can start to use the application.