Normally, you go into "edit account" and there's a box for you to enter your favourites. It looks like Brad took it down again, so he must be working on some part of the site that messes with it. Check back later, or look in System Notices for word that the feature is back online.
You should still be able to do it from each user's comics pages (there's a link below their username), or just copy the relevant link below into your address bar and change your username to the username you want to add or remove.