Before setting up (or after setting up) Google Apps Mail access with Microsoft Outlook, you must do the following:

1. Log in to your Google Apps Mail account with email username and password using a browser (I.E., Firefox, etc.).
2. Go to Settings - Forwarding and POP/IMAP - Enable IMAP
3. Click on Save Changes Button

Note: You might want to also click on "Settings - General" and Select "always use https" in the browser connection section at the bottom. That has no effect on Thunderbird but is a good way to help avoid cross-site scripting attacks* when logged into webmail using a browser.

*type of computer security vulnerability typically found in web applications which allow code injection by malicious web users into the web pages viewed by other users.