Just been on to have a look what it was I did last time: (sorry no idea how to do that screen-grab thing to illsutrate)
-if you look at your basket, (you need 1 or more items in it at this point) then on the left side of the screen there is a big-ish button marked 'update quantity' and then immediately below this is a small box for text, and just above it is says: "If you have an eVoucher code, please enter it here" and then click the 'add' button just to the right of the text box. Then it knocks the value of your voucher off the total cost before you pay.
On my screen this is immedaitely above the 'additional notes' field that you put your code into: I wonder if this is different for different browsers? I'm using firefox fwiw.