Facebook app creation app domain error

Facebook changed their user interface for developer apps, so this can be tricky to find. For future people visiting:

1. Click Apps and then select your app.
2. Click the Settings button on the left side of the screen.
3. In the Basic settings, click the Add Platform button below the settings configuration.
4. Select Website in the platform dialog.
5. Enter your URL (localhost works here).
6. In the App Domains text input, add your domain that matches the one in the URL.
7. Save your settings.