I've answered this question enough to warrant adding it to our FAQ list now. There are a couple of pending comments with the question again, so I'll just answer it here for everyone and add it to our FAQ.
Click on "My Account" at the top of any EarthLink page. Log in there with your primary account (might be your only one if you have never done this). Then click on Profiles. Most accounts allow up to eight profiles (mailboxes) with no extra charge.