if you are using tomcat6 .
the default installation directory will be:
c:\program files\apache software foundation\tomcat 6.0.
in this directory you should find the lib(libraries) directory there you will find servlet.jar or tomcat-servlet.jar and also the main containers responsible for running of servletengine.
extract the jar file and set the class path to the extracted directory or just copy the jar file to the webapplication you building.