I assume you are currently earning miles and not points in your Marriott rewards account right? This is most likely the case since this invitation is probably targetted. If not, you can change it to miles (assuming you prefer that over points...I recommend points) in your profile.
To register, you should either click on the link in your email invitation (if that's how you received the invite) or login to your Marriott account and go to the Promotions section to see the current list of promotions that you qualify for.
I'm guessing that most likely the invite was sent from the airline. If you are a 'points' earner with Marriott, the only way you can register for the Megamiles promotion is to switch your earning preference from 'points' to 'miles' and pick your airline. You will then be able to register for the promo. You cannot earn points and miles at the same time. It is only one earning preference and you get to make the choice. You can switch earning preference if so inclined.
For more about the miles program, see here: