For every purchase you should first add the products you want to buy to the cart with the "Add to Cart" button. When you are finished, you are going to the cart button on the top right corner and click the checkout button. Fill out the checkout information for the delivery, shipping method and payment and finish the order.
The unique code you are looking for is 9-digit number on the side of the box, under the barcode. On the label is written UNIQUE CODE. With that code and the other information you fill, you can register your product for warranty.