If you tried to get a key for your localhost, you might get the following error when you tried to load the map:
In order for a key to work on your localhost, you need to register an unique host name. So, go back to the registration page and pick a really unique name. Pick anything you know will be unique. Then edit your /etc/hosts file and add your unique name to localhost. For example:
127.0.0.1 localhost myrealuniquehostname
Assuming you did everything else correctly, you should be able to see the map display without the pop up error above.