Californian, when you login to Marriott Rewards, click 'Marriott Rewards Home.' Then you'll see a splash page. The top half of the page has a photo of a beautiful island looking property. On top of the photo is the word MEGABONUS in a good sized font. Click on the photo itself, and it will take you to the registration page for the fall megabonus. It didn't come out and grab me right away either.
If you're not seeing the 2012 megabonus program with the dates Sept. 15 2012 - Jan. 15 2013, I think you should definitely call Marriott. I believe that with the megabonus program, we are NOT all necessarily offered the exact same promotion, but I do believe we all each get some kind of customized megabonus promotion, meaning there should be something new out there for you.