In order to use the Google Reviews add-on, you'll need to generate a key for the Google Places API Web Service, and enter it into the provided dashboard settings page.
You can get a key at the following url:
Click "Get a Key", and go through the steps to create a new project.
Once you have the key, return to the Concrete dashboard, and go to the Google Reviews settings page. Enter the key in the form field and save it.
You can then add the Google Reviews block to your page.
The block comes with 3 options, including the option to set the number of reviews to display, the number of characters, and the Google Place ID.
In order to get your Google Place ID, this tool will allow you to search for your business, and retrieve the place id: