To redeem a code on the Mac App Store:
Click Redeem from the Quick Links section located on the right of the Mac App Store.
Enter your redemption code into the text field exactly as it appears on your gift card or gift certificate. You may be prompted to sign in with your Apple ID and password.
You will receive a thank you message.
Your Mac App Store credit balance will be displayed in the Quick Links section. When you redeem a code on the Mac App Store, your store credit balance will be updated for both the Mac App Store and iTunes Store.