Issue
iOS 6 has introduced a bug and many users are affected by this problem. Especially those who bought the item in the US and are outside of the US right now. This bug gives you an error message and does not allow you to download apps or update. The error message is:-

Your account is not valid for use in the xxx Store. You must switch to yyy Store before purchasing.

xxx and yyy represent countries and have been used for illustrative purpose.

Resolution

1. Using your iPad or iPhone, click the App Store icon.

2. When in App Store, press Featured tab at the bottom of the screen.
3. You will see your Apple ID. Press it and a pop up will show up. 
4. Press View Apple ID and provide your App Store password. 
5. Your ID will get either get switched to the U.S. account or the country where the account was created. Otherwise you will see an option to change the Country/Region.


http://www.survivalguide4idiots.com/ios-6-bug-your-account-is-not-valid-for-use-in-the-xxx-store-you-must-switch-to-yyy-store-before-purchasing.html