Here's some information from the HubPages official blog: http://blog.hubpages.com/2011/03/hubpages-ad-progr
The program is by invitation only right now. They'll be inviting groups of users to participate in the program from time to time. They plan to give everyone access to the ad program by June. So basically we just have to wait until we get an invite or until the program is open to everyone in June.