You will need to provide a database for your ${PN} 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.php

and provide the credentials required for the database access.

Note the credentials of the administration user you will generate
during the installation procedure.

For your own security you will need to remove the installation tools
by running

rm -rf ${MY_INSTALLDIR}/install*

Enjoy ${PN}!