1. Seller Engine Plus will need you to authorize it for MWS in order to access needed data from the Amazon API so after logging in the MWS Registration window will pop up:
2. Click the "Register to MWS" button, this will take you to the Amazon website where you will be asked to log in. After that, you'll need to read and accept the Amazon MWS license agreement:
3. Complete the authorization process by following through the simple steps until you see a screen that says "MWS Enabled. Thank You. You can now start using SellerEngine Plus."
To double-check that Seller Engine Plus is now able to retrieve data from Amazon make sure there's a green checkmark icon in the bottom-left corner of the Seller Engine Plus window: