Some promotions are available as a discount code sent to you via email. These kinds of promotions are tied to your DoTravel user account and, therefore, can only be applied when you are logged in. Simply add your selected tickets, tours or activities to the cart and proceed to checkout where you will have space to enter your code.
The second option is promotion codes distributed via social media or awarded in DoTravel online contests. If you receive such promotional code, just type it in at the checkout to see your savings (Please note, promotions can only be applied to new bookings, not old ones).