If it's in the nutrition tracker that you are trying to add that, then this page has info on how to do that: www.sparkpeople.com/community/help_answer.
It is also a very similar process in the recipe calculator on the recipe site. When you click on the add an ingredient button, you will see the link to enter a food not listed. Once you enter and save the item, you can choose it from the drop down menu of your favorites in the add an ingredient box.
I hope that helps!