I hope my answer will help anyone who want to set up a buy x Get y free on Magento:
1st. Go to Promotions > Shopping cart prices rules
2nd. Click on Add new rule
3rd. Define a name, a short descritpion, select website and customer groups
4th. Define if your promotion require a coupon to be activated or not
5th. Define a period
6th. Conditions : nothing to set up here
7th. ACTIONS :
a> Select "Buy x Get Y free"
b> Discount amount put "1" (that means 1 prodcut free)
c> Maximum qty discount apply : Select how many times in the order this promotions can be applied
d> Discount qty : Put "1" for a Buy 1 get 1 Free or put "3" to apply a buy 3 get 1 free
8th. Rule to apply to cart items : clik on SKU and type the sku of the product concerned by this promotion.
So basically, if you have set up a Buy 3 get 1 free promotion for example, you customer my need to add 4 products in his cart, then he'll get a 100% on one of these products.
Feel free to ask for more details.