Hover over it with your mouse and click Edit Your Website and Mobile App List

The next screen will allow you to "List all the top level domains and/or mobile apps on which you plan to display banners, widgets, Special Links, or other ads from Amazon Associates". You can add up to 50 apps and sites.

Now type or paste your site url address or app url(s) into the box and click Add. I personally prefer the copy the url and paste it method to avoid any mistakes.