Call 1 800 321 7396, bypass robot by saying "agent" twice, talk to the nice folks at Marriott Rewards and ask to be registered for the MegaBonus.
To increase the odds of getting the type of "personalized" MegaBonus you want, browse thru this thread
You shouldn't have any problem, as we haven't had any reported cases of anyone not getting a bonus, if they ask (may not get the one you want).Good luck
I had to do this as well. Joshm registered me for mine. My new problem is the hotel where I am staying hasn't posted any of my nights. I tlaked to the front desk staff and the gentleman said there are about 85 identified properties that are backlogging stays for one reason or another, but it is an identified problem and will grant the points and nights when the system problem is resolved. I don't know yet whether I want to just contact Platinum Support or just wait it out, but I'm here for another month so I'll wait until my stay is concluded before I try to have them manually logged.