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.