I just brought an iTunes card and I redeemed it from iTunes instead of the app store. I'm trying to download the imovie'11 update. On the app store it shows no sign of me having 15 dollars on my account and so I can't download the app.
Welcome to Apple Support Communities
In theory, you can redeem an iTunes gift card on iTunes and use its credit to purchase content at the Mac App Store without any problem.
Open the Mac App Store and check next to "Account" that the credit has been added. If not, try to redeem the code again
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