Amazon Payments Sandbox Setup
If you have not yet set up your Amazon Payments Live Setup yet, please do so before creating the Sandbox.
When logged into Amazon Seller Central using your normal Amazon username, look in the top for a drop down next to the Search box that looks like this:
Click the dropdown menu and choose Amazon Payments Advanced (Sandbox View), like this:
On the new page that loads you'll see in the top left a nav item called INTEGRATION. Under there you'll see MWS Access Key. Click on MWS Access Key.
On the resulting page you'll be presented with all of the information you need to fill out the form fields you'll find on the Downloads → Settings → Payment Gateways for Amazon.
Once the page reloads you'll find a new set of fields related specifically to Amazon as seen below.
Click the Save Changes button at the bottom of this page and then go to Downloads → Settings → General tab and make sure your site is in Test Mode.
Once the above items are all set up, create an Amazon Test User and you can begin testing your store.