In order to redeem a voucher you need to be logged in to your account. At the status area where you can see your balance you will see “Redeem Voucher” klick on it.
Another way would be: klick on the tab "My Account" on the top navigation of the page. After your "My Account" Area is loaded you will see a second navigation bar, the third option is labeled "Vouchers" please klick on it. On the next page you will have a field where you can put in the voucher code. After doing so, please klick on "Redeem Voucher" right to the input box.
A popup will tell you if the redemption was successful.