A:Answer First off, while you "should" receive your code in via email, it is also printed on the bottom of your receipt for any eligible product. It has an activation code there, as well as a link to enter. You should be using Chrome or Firefox as a browser - anything else creates problems or plainly doesn't work - but the link doesn't work (rather, it doesn't route you anywhere but the homepage anyway).
Second, you need to have a shutterfly account.
Third, once you are using the proper browser and signed into your account, in the top right section of the home page, you will see a spot that says "Hi, YOURNAME". Click there. Then click on "Settings" (counterintuitively, this is where you use the code).
Once on the Settings, enter in the code right near the top of the body of the page, where the open bar allows you to enter a code (then click the arrow to the right to submit). If you have typed it in properly, it will show green text under the box where you entered the code, saying that the code has been added (example pic shown).
You may then click on "my promos" right under the very same box where you entered the code on the settings page, and see (there are likely multiple) what promos are applied to your account, including this one.
FINALLY - and this is important - you CANNOT combine more than one promo at a time, sadly. For example, if you have 4 different $25 codes, you cannot use them or "stack them" as shutterfly prevents you from doing so. You can only use them one at a time.
Overall, a terribly executed promotion.
