To use your selling account with Amazon MWS, you need to go to http://developer.amazonservices.com and click Sign up for MWS. Sign into your staging account and follow the on-screen instructions. The last page of registration will display the Merchant ID and Marketplace ID for the staging account, plus the additional developer credentials you need to make successful MWS requests.
Note: To be eligible to use MWS, sellers must have at least one of the following: a non-individual Selling on Amazon account, an Amazon Web Store account, a Checkout by Amazon account, or an Amazon Product Ads account. Sellers with only individual Selling on Amazon accounts are not eligible to register for or use MWS. If you are an individual seller and would like to use MWS, you can become eligible to register for MWS by upgrading to a Pro Merchant seller account from the Amazon Services Selling on Amazon page.
More at developer.amazonservices.com
Sign in to you non-individual seller account:
Choose appropriate option and click “Next
Accept Terms of Conditions and click “Next”.
Save your Amazon MWS parameters: