Facebook has changed since last year and has now created a new procedure in adding apps to Facebook pages. In order to add a Facebook app to your Facebook page, you need to follow the link (changing YOUR APP ID and YOUR URL):-
To set up our App on your Facebook page, you have to insert our app id 560923594002724
Then open the link and it will give you the option to choose from the the pages you are hosting.
Choose the one you need and click the Add Page Tab button at the bottom. The App will then be shown on your page.