MapQuest geocoding library
To be able to use MapQuest geocoding library on your site you will to get an Consumer key. First you need to sign up (or sign in if you already have an account) at MapQuest development center. You may do so at the following URL:
Once signed up, create a new app. You may leave Callback URL empty.
Now make a note of your Consumer Key.
Now go back to MapQuest settings page on your site and fill in your Consumer key in the appropriate text box.