According to the web site you have to go into an Apple store within 30 days to add AppleCare+:
http://store.apple.com/us/product/S4575LL/A/applecare-for-iphone
-HM
Yes thats one way but usually can just also add it to your shopping cart and be auto-enrolled thats how I did it for every previous iphone and their site http://store.apple.com/us/product/S4575LL/A/applecare-for-iphone still indicates the option to add to cart