When creating a promotion, you will have the option to discount services or gift certificates, so add-ons cannot be included in a promotion. However, there are a couple workaround options.
The first would be to create a service on your service menu that lists the service and add-on in the title and description and set the price to be the full price of both the service and add-on.
For example, let's say you want to offer a free Aromatherapy add-on (originally priced at $10) when a client books a Swedish massage service (originally priced at $60). You could create a service on your menu titled Swedish Massage with Aromatherapy and within the description list that the service includes a free Aromatherapy add-on when booked through the promotion. You would then list the price of the service to be $70, the total of both items.
When you create your promotion, you can choose the new Swedish Massage with Aromatherapy service and discount the total by $10. That way the client would be able to purchase at a discount, and you will see on the schedule that Aromatherapy should be included.
These links will walk you through settings up the above option:
Another option would be to change the price of your add-on on your service menu. That way when a client books as normal, they will see the discounted pricing for add-ons and will be able to book them. Once the appointment is booked, the add-on will remain at that pricing. So you can then change the price back to the normal rate whenever you'd like and it will not affect appointments that were scheduled during the time of the discount.