To add an item to your set menu is simple. From the set menu page you will edit the menu off your choice and under the long description you will click the yellow button titled Add an item.
Then select the item, you may have to look for the correct category to find the item you wish to add. You will select that item, and then proceed to add the other items that will be part of the set menu.
For example, if you have a menu that is quiche and salad you will select for quiche and salad to be displayed in the set menu, and define the set menu price. You may then want to create a set menu for quiche salad and dessert. Giving site visitors an idea of prices and common menu combinations.