The registration code input button is the very top and different size from all the other button, which I find confusing at first since I was looking for it too. However if you have not yet received your physical copy, you wont have the registration code to put in. My brother ordered the physical copy and can't play too because it's not yet been received. I tried live chat for him for hours and in the end got the message : no agent available to try later TWICE!!

Lesson is NEVER BUY PHYSICAL COPY FROM SE. You pay more, wait longer and can't play.