If you've received a voucher code, you can redeem it from a Purchase Credits screen.

When prompted to add more credits, press the blue I have a voucher code button

Enter the voucher code you received in the box where prompted, then press the green Continue button

Important: Voucher codes may have terms and conditions, and may not be available for all accounts. Voucher codes will only add credits to your account of the type offered by that voucher. For example, voucher ABC123 will only give you 500 e-mail credits, even if you pressed Purchase Credits from the SMS section.