In a ISPConfig setup, postfix is not connected to the database. It is a file based setup.
1) Follow the configuration steps described in the perfect setup guide for your installation. The Guides are linked in the documentation page on the ISPConfig website.
2) add these two lines at the end of your main.cf file and restart postfix:
virtual_maps = hash:/etc/postfix/virtusertable
mydestination = /etc/postfix/local-host-names