If you're starting Outlook for the first time, the new account dialog will appear automatically. If you are configuring an existing setup, go to Tools --> E-Mail Accounts using the menus at the top of the window. If Outlook was formerly configured to use POP3, select View or change existing e-mail accounts, and delete any SeattleServer-hosted accounts that currently exist before continuing. At the following window, select Add a new e-mail account and click Next >:
If you're starting Outlook for the first time, make sure you choose the "Internet Mail Only" configuration unless you also use an Exchange server. The new account dialog will then appear automatically. If you are configuring an existing setup, go to Tools --> Accounts using the menus at the top of the window. If Outlook was formerly configured to use POP3, remove any existing SeattleServer-hosted e-mail accounts before continuing. In the Accounts window, click the Add button, and then select Mail to bring up the new account dialog. Enter your name here and click Next >.

Enter your E-Mail address and click Next >:

Change the My incoming mail server is a _____ server option to IMAP. Enter imap.yourdomain.com as the Incoming mail (POP3 or IMAP) server, and smtp.yourdomain.com as the Outgoing mail (SMTP) server, substituting your domain name for yourdomain.com. Click Next > when finished.

On this page, enter your E-mail address as the Account name and your password. Do NOT check the SPA box. When complete, click Next > a couple times then Finish, and the wizard is finished.

You will now be back at the Internet Accounts window (go back to Tools --> Accounts if you accidentally closed it). Select the account you just set up, and click Properties.

On the Servers tab, check the box for My server requires authentication. You can now click Okay, and close the accounts dialog.

Your account is now set up and ready to go!
Old Mail Tip:
If you were using Outlook with POP3 previously, all of your old mail is still in Outlook Today/Inbox. It is recommended that you move this into your IMAP Inbox to have all of your mail in one place, and so that it's stored in a safe place. To do this, simply go into Inbox under Outlook Today, select all your mail, and drag it into the Inbox for the account you just configured. You should also move any old folders from the Outlook Today section into your IMAP account. To enable storing your Sent mail on the server, please see Microsoft Knowledge Base Article #198854.