How do you redeem an iTunes Gift Certificate?

I have already added the card to my account, and at the top of the store page it says that I have .00, but when I bought something that amount didn't go down, so I looked in my recent purchases and it said they had charged my credit card, instead of the gift certificate.
Why didn't it use my certificate money?
i read through the Apple website, apparently you cannot buy apps in Canada with gift certificates.