Signups are now handled at Comic Dish, where MAX is moving to. Register at this page. Once you have an account, go to your new MAX Account and fill out the information on your homepage and characters. Apply for approval, and if you meet the requirements, you'll be ready to sign up for MAX exchanges.
Just to be clear, you must meet these requirements:
1. You have a personal website that has your art and characters on it, or an image hosting service such as DeviantArt.
2. You are not on the blacklist.
3. You have at least one character of your own invention and some publicly available images and information on that character.
4. You have the ability to have one decent picture created and online in two weeks.