I encountered this oddity in Sizzle 0.1 today. After launching the app I found that the 'Add Chapter' button was greyed out. After quite a lot of head scratching and poking around Sizzle, I finally discovered that it was caused by having the 'Use Custom Chapter Times' checkbox selected under the 'Other' tab (from a previous authoring session) without having a chapter selected in the 'Chapters' tab.