You need to register your app at developer.facebook.com/apps.
Set a canvas URL (which is the url where your app is hosted) [Usually the URL of your php script]
I decided to host on localhost.
The namespace becomes the URL at which your app is accessed
(
My namespace was sim_photodown and my app is accessed at : 
https://apps.facebook.com/sim_photodown/
)

Settings for the app

App ID/API Key
140450869386382

App Secret
APP_SECRET
[ This is not a secret if I disclose it :) ]


Canvas Page
https://apps.facebook.com/sim_photodown/

Canvas URL
http://localhost/photodl/

Secure Canvas URL
https://localhost/photodl/

Canvas FBML/iframe
iframe

Contact Email
[email protected]

Support Email
[email protected]