To add an aditional e-mail address into a separate INBOX, you first need to create what is called an Identity. Once you have created seperate identities, you can switch between them to check the seperate INBOX's.
To create a new identity, first open Outlook Express.
click "File", "Identities", "Add New Identity"
Type the name of the new Identity.
Click "OK", then "Yes" if you want to switch to that new Identity. Outlook Express will now be loaded with this new Identity and you can enter the information that the "Internet Connection Wizzard" asks for.