To get a FaceBook App ID visit https://developers.facebook.com/apps - Log in with your facebook details and click the Create New App button. Supply the details asked from you and you will receive an App ID / API Key - Enter the APP ID / API Key here.

Note that you also need to specify the URL of your domain name. You do this by clicking on 'Website'. You will see the 'Web Site' option on the page where your APP ID is displayed.