To create an Open Item, login to the backend and go to the Menu.
Choose where you want the Open Item to appear in the Menu and add a new category and item.
Name the category and menu item accordingly and set the price to 0. On the same row on the right side, find and click Details.
In this pop-up, you can apply a Custom Tax Rate of 0% or No Tax Profile, if you chose. Below that you will see Open Item and a drop down box. Make the appropriate selection and then scroll down and select Apply.
Be sure to also hit Save. Once you Reload Settings, you will be able to see these changes.