When you register an application please note that Amazon only supports HTTPS.
1. Go to http://login.amazon.com/manageApps and log in to your account or create an account if you do not have one. After logging in you will be automatically redirected to https://sellercentral.amazon.com/gp/homepage.html.
2. Press "Register new application" in the top right corner of the page
3. Fill in the necessary fields and click Save:
4. Go to the Web Settings section and click Edit.
6. Allowed Return URLs – this parameter must be copied from Redirect Url option in the extension settings in Amazon section:
7. Save the settings.
8. Set the values for Client ID and Client Secret in the extension settings in the admin panel of Magento®.
Amazon Wiki URL: http://login.amazon.com/website