The app is not available to general public yet. To make to app available to every Facebook user, you have to submit the app for review. After a review, Facebook makes the app available to every Facebook user. You can find more information on the review process by clicking on App Review in the left navigation menu of your app's dashboard.
The review process may take some time, so for the purposes of this sample, you can specify some Facebook users as Developers or Testers. Only the users specified here can use this app to log in with Facebook until the app goes public. To do this, click on Roles in the left navigation menu of the dashboard and specify the required Facebook users as Developers or Testers.