Once you have saved your Amazon.com MWS keys in our API section https://app.fbawizard.com/user/api (under MWS US) you will see a flag in the top left (assuming you have already saved UK keys) where you can toggle between UK & US searches.

If no UK keys saved, you can just go to https://app.fbawizard.com/scan and the US retailer list will be there.