You can find Merchandise in the Other tab of the Sales Interface.
Merchandise can be added to the basket in a similar way to seats. If you know the name of the merchandise item you are looking for, enter a partial name and click search. If you would like to add an extra item of a merchandise type already in your basket, leave the Stock item name field blank and click In Basket.
To add a merchandise item to your basket, adjust the quantity by typing into the Qty field or by using the + and – buttons and then use the add to basket button to select.