Unable to split events in iMovie 11

Hello! I've created few iMovie projects, but most of them are more than 120 mins in length. I've tried splitting them into two events as describes in the manual, but whenever I highlite the clip where I want the project to split, the command "Split Event Before Selected Clip" is grayed out. It occurs whether I tried doing it in Project Library or Event Library. Any help or suggestions will be appreciated.