Faces is not a standard part of either Eclipse or Tomcat. Consequently, specific Faces resources must be obtained and installed in Eclipse and made accessible to tomcat. The process is extremely brittle and can result in catastrophic failure such that none of your applications, Faces or otherwise, will work. And fixing those problems may require the intervention of an administrator. So, be sure to follow the the procedures closely and be sure that your have thoroughly tested your application on your own development machine before porting to a production server.
Eclipse
This discussion will assume Eclipse Helios IDE for Java EE Developers and Tomcat 6.0.28 as the JavaServer. JavaServer Faces (JSF) is now a built-in part of Eclipse, although several additional libraries are needed for some applications. Those libraries are created or downloaded as part of creating a new JSF project; thereafter, they may be added by selection to subsequent project.
Details of tht process are described in the lesson on Building a Development Environment.