You will be able to add promotional/discount codes in the 'Apply Discount Code' field in your cart before you proceed to checkout, or during checkout in the 'Payment + Confirm' step.
Once you have entered the code, click 'Apply'. You will see a green banner at the top of your cart that confirms that the code was successfully applied.
Please note that only one promotion can be used per order. However, it can be used in conjunction with the free shipping promotion for orders above $300 as this promo will automatically apply.