Hi Anne Marie,
Go to the settings and switch back to US American and the next time you open up the Store it will show Sony Reader Store in America. You must of course be registered with the US store before you can do any purchases, in most cases your own address will work if you exchange zip, state and country fields to valid US choices. (Hint: look up a multinational company and "borrow" their zip and state).
Good luck!
ArcticBoy