To get your Amazon MWS details you must sign up to MWS. To register for Amazon MWS, sign up using the appropriate link below:
Click Sign up for MWS.
Log into your Amazon account.
Select I want to access my own Amazon seller account with MWS as shown by the first option in the image below.
Accept the MWS License Agreement.
Your Access Key ID, Secret Key ID, Merchant ID and Marketplace ID are displayed, example below.