Promo codes can only be redeemed using the US app store. The instructions below shows how you can redeem promo codes for people who have non-US iTunes accounts.
Step 1: Login iTunes and go to “iTunes Store -> App Store”.
Step 2: Change to the US app store.
Click on the flag at the bottom and selecting the “United States” flag.
Step 3: Create an US account.
If you have a gmail account, you can use the same email address by adding a “.” into your email address. (e.g. email@example.com, firstname.lastname@example.org are the same email address).
Step 4: Look for the “Redeem” button at the bottom of iTunes.
Step 5: Press redeem button and enter the promo code that you received.
There is no need to provide credit card information. Just go to the next step.
Step 6: Then iTunes will display the following downloading status for the application.