The website for the Spring 2013 offering of CS188 Artificial Intelligence is hosted at http://berkeley.edx.org. Once on this site, register for an edx account, and then enroll in the Spring 2013 offering of CS188. Be aware the sites hosts many versions of CS188, including multiple offerings for Spring 2013. Make sure you pick the internal Berkeley course, which is called CS188 (as opposed to the global offering, which is called CS188.1x).