You can use sample code bellow since version 4.0:

        // Get current shopping cart
        ShoppingCartInfo cart = ECommerceContext.CurrentShoppingCart;
        if (cart == null)
        {
            // If current shopping cart hasn't been created yet -> create new empty one
            cart = ShoppingCartInfoProvider.CreateShoppingCartInfo(CMSContext.CurrentSite.SiteID);
            // Associate new shopping cart with the current session
            ECommerceContext.CurrentShoppingCart = cart;
        }

        int skuId = 158;
        int quantity = 2;
        int[] options = { 159, 160 }; //SkuIDs of product options

        // Ensure shopping cart is saved to DB
        if (cart.ShoppingCartID == 0)
        {
            ShoppingCartInfoProvider.SetShoppingCartInfo(cart);
        }

        // Add item to the shopping cart
        ShoppingCartItemInfo product = cart.AddShoppingCartItem(skuId, quantity, options);

        // Update shopping cart item in database
        ShoppingCartItemInfoProvider.SetShoppingCartItemInfo(product);

        // Update product options in database
        foreach (ShoppingCartItemInfo option in product.ProductOptions)
        {
            ShoppingCartItemInfoProvider.SetShoppingCartItemInfo(option);
        }


If you use some older version, please use following sample code:

        // Get current shopping cart

See also:  

Applies to:  Kentico CMS with E-commerce module