Scroll down on the page till you see, "Amazon MWS Developer Permissions". Then click the "Authorize a Developer" button.
Step 3: Add Third-Party Application (WatchFacts)
Click the bubble that says, "I want to authorize an application to access my Amazon seller Account...." then under application name enter: "WatchFacts Amazon MWS" then under, application's developer account number enter the following: 2755-8444-9118 . Then click the next button.