Step 1:  Click on the right link according to your Amazon Seller Marketplace:

US:
https://sellercentral.amazon.com/gp/mws/registration/register.html?signInPageDisplayed=1&devAuth=1&developerName=3DSellers&devMWSAccountId=7911-5228-2199

EU:
https://sellercentral.amazon.co.uk/gp/mws/registration/register.html?signInPageDisplayed=1&devAuth=1&developerName=3DSellers&devMWSAccountId=6913-8884-1100

Step 2: Read the terms and condition and click 'Next'

Step 5: Send us the Seller ID, Marketplace ID and MWS Auth Token:

If you have any questions, we're always available on the chat box within the platform.