Step 1: Accessing Your Amazon MWS Seller/Merchant ID & MWSAuthToken:
To get your Amazon MWS Seller ID & MWSAuthToken, you must sign up with MWS. To register for Amazon MWS, sign up via HTTP://DEVELOPER.AMAZONSERVICES.COM Click Sign up for MWS and then log in to your MWS account – or register if you don’t have a MWS account (you must have a Amazon Pro Seller account to register with MWS). Note, if it’s your first time registering, on your first login it may send you to your Amazon seller account; if this happens, just go back to the “Sign up for MWS” button using the link above and it will redirect you to the page displayed below:

Agree to the terms on the next page to continue.
Copy & save your ‘Seller ID’ and ‘MWSAuthToken’