As of late 2011, Facebook has begun to phase out the View App Profile button you may have used to connect Facebook Apps to fan pages.

The replacement technique is rather strange. You need to type a specific URL into your browser and fill out the following:

https://www.facebook.com/dialog/pagetab?app_id=APP_ID&redirect_uri=TAB_SOURCE_URL

If you provide the correct parameters, you will receive the following dialogue box:

You can read more at Facebook’s official documentation below.
https://developers.facebook.com/docs/reference/dialogs/add_to_page/