The process of adding or redeeming AppSumo codes is highlighted in this short video: 

If you can't view the above video please follow these steps to redeem your AppSumo codes: 

1. Login to your AppSumo account via the Chrome extension. Click on the up arrow (^) at the bottom left of the Chrome extension popup.
A new window will open, click on "Change Subscription"
Then click on the Enterprise plan:

2. You will be given the option to enter your code(s). Once the code is added, please click on Apply.

3. Adding more coupons/stacking codes: You can now apply more codes (if you are stacking them) by going to Subscription and adding more coupons in the text box (as shown below). 

Each time you stack a code, you will receive an additional 200 credits/month.