If you are looking to claim a voucher code (towards adding more data to your monthly allotment, for instance) you can do so from the My Account page:

Scroll down to the Account Status line and you will see it to the right:

For complete instructions check out this article.

If you are looking to claim a promo code (towards a discounted subscription, for instance) you'll need to do so from the Upgrade page:

For complete instructions check out this article.