Not sure what Megabonus you enrolled in but whatever Megabonus program you get can be found in this way:
Go to www.marriott.com and sign in to your account on the upper right. After you have signed in there is a dropdown menu. On the bottom of that dropdown menu is a category for 'Promotion Central'. Click on 'promotion central' and the menu expands to say 'Promotions for you' and 'see all promotions'. Click on either of these and you will be taken to a new page that lists the promotions that are available to you. If you are registered for the promotion there will be a line that says YOU'RE REGISTERED. If you are not registered, you can click on the promotion and sign up for it.
Hope this helps. Happy travels!