To change your Marketplace region, all you need to do is go to the Settings menu of the XBox 360, select System Settings and then Locale.
Change the Locale to USA (or any region of your choosing) and save the change.
Your Xbox 360 should now be showing USA based apps.
If you are still having problems, please email us at firstname.lastname@example.org, and try to include any errors and a full description of the fault.
We'll get back to you as soon as possible.