The appliance obtains its IP via DHCP. The virtual console will display it at the top of the screen:
Citadel virtual appliance - (C) 2007 http://www.citadel.org To log in to Citadel, point your web browser to http://192.168.1.22
Simply point your web browser there and get started. The default administraor has the username Citadel and password citadel. Obviously you will want to change the default once you log in. After you have logged in once with webcit, you can connect to that account with your favorite mail client via POP, IMAP or SMTP. A more thorough Getting Started may give you aditional information.