Hi Edmond,
You need need at least 30 reviews within the last 12 months, and the ratings need to be 3.5 stars minimum on average. BUT: Having these ratings doesnt garantee anything. Usually my experience is that having them in place will take about 6 weeks before they show up in the ads.
On https://services.google.com/fb/forms/SellerRatingExtensions/ Google says they should be included automatically. PS: The form on the page is for disabling the stars.
In The Netherlands Google adwords provide a very good support desk for all questions regarding your ads.
Good luck!
Tymen