Some editions (especially multi-packs for classrooms) include a separate cardboard card tucked between the back cover and the last page. This card has the code printed on a sticker. Do not throw the card away with the receipt.
If you bought a used book, assume the code has been used. Penguin Readers unlock codes are single-use only . Once a code is redeemed, it cannot be used again.
The unlock code adds real learning value, but technical limitations and strict usage rules hold it back from being excellent.