If you want to add your app to a Facebook page, you will have to do the following.

1. Prepare your app

Make sure your have has valid URLs to point to from the little thumbnail in the Facebook page. Add an image that will be displayed. To fit the image into the area on the Facebook page it needs to be 111 x 74 px.

add_app_facebook_page

2. Add the app
To add the app to the page you need to run the following link:

https://www.facebook.com/dialog/pagetab?app_id=[APP_ID]&next=[PAGE_TAB_URL]

The app id can be found https://developers.facebook.com/apps/
facebook_app_your_app
3. Add page tab dialog
The page tab dialog will show all the pages you have admin permissions to.
add_page_tab_facebook