Some school bookstores sell/provide prepaid access keys to students, to allow students to directly use financial aid. (Note: Most students on financial aid still subscribe via credit card at zyBooks.com; see this article). A few schools include the subscription cost in the student's fees and thus provide access keys to all students.
A zyBook prepaid access key is 10 characters and starts with zy, like zy123abc45 (just an example).
To use an access key, students first go to a zyBook's home page. If unsubscribed, a price will appear, followed by a payment info area. That area may have an access key box:
Upon entering a valid access key, the price will drop -- usually to $0, so no credit card is needed.
An access key is specific to a particular zyBook code, and cannot be used for a different zyBook code (including for a different term).
NOTE: The access key box only appears if that particular zyBook code has access keys issued.
Sometimes the item provided by the bookstore has the zyBook prepaid access key in hard-to-find spots (like another paper stapled to a receipt, or on their website that students must log into).
If your access key does not work, or no access key box appears, your bookstore probably provided a wrong key (different class, previous term, etc). Contact firstname.lastname@example.org with info (zyBook code, access key, instructor/class name, etc.) and we'll try to get you learning ASAP.