A promotion can be added while detailing items on an order, BEFORE the order is closed. While at a screen similar to the one below, at the same time you would scan a barcode on an item, scan the barcode for the promotion:

This will apply the promotion to the order, and you can continue detailing the remaining items or Close the order if you are done!

If you want to add the promotion to an order that is already closed but still in processing, you can select Add Item and then scan the promotion: