You need to add the URL to your app:
- Go to the app, you want for user login, on the Facebook Developers page
- Click on the settings tab
- Click add platform
- Select Website
- After selection it will ask for some details such as URL for your website which uses login with facebook feature, fill the form and submit it
That's all and you are done. Make sure that the app's URL is the same from where you're logging in.