Once you've logged in and got to the bit of the screen where it says "Voucher code", it should just be a case of entering (or cutting and pasting) the code to the box and click on "Use voucher".
As soon as you do that, you should get your 10 credits plus a confirmation email.
Let me know if you still have problems.
Dave
Added, don't click purchase or choose a payment method. Just the code is required.