Method 1: From the Item's Cover
Go to the Digital Bookshelf section of your account
Find your item and select Click to Reveal on the cover
Select Access
Your code will either:
Appear immediately for you to copy, or
Show up in a popup box in the top-right corner (click the hyperlink in the popup to reveal your code)
Once you have your code, you can redeem it on the publisher's website.
Method 2: From Your Library
If Method 1 doesn't work:
Click the Back or Library button in the top-left corner.
Find your item's box in your library
Click the three dots (⋮) in the top-right corner of the item
Select View/Reveal Access Code