That class is typically provided by your application server’s implementation of the Servlet API, in Tomcat 5 this file is servlet-api.jar in the common/lib dir. You likely need to add this as an external JAR via the Project Properties > Java Build Path > Libraries > Add External… button.
The reason you typically don’t need to drop these JARs into your project’s WEB-INF/lib JAR is because they are provided by your application server, so you don’t want to be deploying these ‘system-level’ JARs with your app, but you do need them for compile-time requirements… so that’s why we use the External JAR thing to hook them into your project.