This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
You probably downloaded an official Tomahawk release from the Apache MyFaces website. The fieldset component you're trying to use is not (yet) in the Tomahawk release. Tomahawk uses a separate "sandbox" project to test new components. A new component may eventually be moved from the sandbox to the Tomahawk release, but there are no guarantees. So I wouldn't advise to use these components in a real life project. But if you want to play around, you should download a recent build of the Tomahawk sandbox, which can be found at the website of Irian, the company that originally donated the Tomahawk components to Apache.