The instant win game runs from Monday, February 2, 2015 through Saturday, February 14, 2015 at participating restaurants. You actually have to enter the restaurant for a chance of winning, but you don't have to buy something or even order to be selected. You can find more details on the official rules page on the McDonald's website.
In case you're wondering, a total of 100 free meals will be given at each participating restaurant over the promotion period (with over 14,000 restaurants in the U.S., if all of them are participating, that's a total of over 1.4 million meals given out). Most of those meals/prizes will be given away tomorrow when 35 meals are earmarked to be given out at each restaurants and on Tuesday, where 20 meals are meant to awarded.
Here's the commercial for the campaign that will run during the Super Bowl today: