Just before February 2012, Facebook removed about pages for Facebook apps in favour of creating “community pages” related to the app manually, from the app’s settings page adjusted by admins and developers.

This also means that Facebook “forgot” to add a link to let you add Facebook apps to Facebook pages.

To do so, you can go to a URL based on your app’s API key:

https://www.facebook.com/add.php?api_key=[api_key]&pages

Otherwise, you can try this tool made by @bryngfors on Twitter:

http://stickybeat.se/addapp/