Remember, you'll need to have a Walgreen's photo account, which is quick and easy to create.
Now, I have taken advantage of Walgreen's offers such as these in the past, and found their website gets slammed. That being said, I've had twice when my order would not go through. If that's the case when you try to place your order, understand it is because there are lots of folks like you looking to get in on this deal. Be patient, and try again. If it still doesn't work, then just send them a nice little note via their Contact Us link, or give them a call at 1-866-264-2910, and simply let them know the site would not process your order. I have found that generally, by the next day, I've had a credit posted to my Walgreen's photo account for the item I was supposed to get for free. Just make sure to try your order before the deadline, and send the note before the deadline, if needed.
Now, go get place that order!