Depends on what you want to do.
If you only want to graze around the kiosks at World Showcase, there is no special admission for that - it is included with Epcot admission - but you will need to pay for the food and drink samples. The Eat to the Beat concerts, should they interest you, are included with Epcot admission.
If you want to attend seminars or demonstrations, you will need to purchase admission to those. On the Restaurant board there is a thread dedicated to the Festival. The first post has information on the seminars and special dining events. Many of these seminars and events require separate valid Epcot admission.