From Support

Google Maps requires that you generate a unique API key for every domain name used to display maps. When you're editing your site, we can show you the map without an error because you are on the "realdesignonline.com" domain. If you have chosen one of our free domain options such as "myrdo.com" then when you publish a site, you won't see an error. However, if you have purchased your own custom domain, you will need to create a unique API key from Google.

• Go to http://code.google.com/apis/maps/signup.html
• At the bottom of the page, enter your domain name in the box, including the "http://"
• If you don't have a Google Account, you will need to create one. If you have a Gmail address, you already have a Google Account

After you have created your API key, you'll need to paste it into the map widget. Click on the map widget's settings, and copy the key into the field. After you re-publish your site, you will no longer see the error message.