In order to use the Sweet Yelp Reviews add-on, you'll need to signup as a Yelp developer here:
Once you have a Yelp developer account, you can retrieve your API keys here:
https://www.yelp.com/developers/manage_api_keys
You'll need all four keys, plus the business ID for your business.
You can get your business ID from the url to your business's page on Yelp. For instance, if the url to your business on Yelp is:
http://www.yelp.com/biz/starbucks-saint-petersburg-11
The last part of the url, (e.g., "starbucks-saint-petersburg-11"), is your business ID.
Once you have the keys and your business ID, you can enter them into the dashboard Yelp Reviews settings page that is created upon installation, and then you can add the Basic Yelp Reviews block to your site.