on this page
replace XXXXX with your app number
https://developers.facebook.com/apps/403XXXXXXXXXXXX/settings/
click add platform below. Choose Website. Add your domain that has to be the same as the one above.
I saw the screenshot that helped me solve my proble here
http://www.ogbongeblog.com/2014/03/unable-to-add-app-domains-to-new.html
