ZK - Open Source Ajax Java Framework

I am getting the following error while running ZK application. Can you please let me know the jar file of the class name org.zkoss.zss.app.MainWindow which I have missed during running of the application. Thank you in advance for your help.

org.zkoss.zk.ui.UiException: java.lang.ClassNotFoundException: org.zkoss.zss.app.MainWindow
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
org.zkoss.lang.Classes.newInstance(Classes.java:78)
org.zkoss.lang.Exceptions.wrap(Exceptions.java:164)
org.zkoss.zk.ui.UiException$Aide.wrap(UiException.java:46)
org.zkoss.zk.ui.metainfo.impl.ComponentDefinitionImpl.newInstance(ComponentDefinitionImpl.java:298)
org.zkoss.zk.ui.metainfo.ComponentInfo.newInstance(ComponentInfo.java:664)
org.zkoss.zk.ui.impl.AbstractUiFactory.newComponent(AbstractUiFactory.java:90)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:600)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:583)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:527)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:559)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:527)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:494)
org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:375)
org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:296)
org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:229)
org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:166)
javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

root cause

java.lang.ClassNotFoundException: org.zkoss.zss.app.MainWindow
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1360)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1206)
java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
java.lang.Class.forName0(Native Method)
java.lang.Class.forName(Class.java:247)
org.zkoss.lang.Classes.forNameByThread(Classes.java:199)
org.zkoss.zk.ui.metainfo.impl.ComponentDefinitionImpl.resolveImplementationClass(ComponentDefinitionImpl.java:344)
org.zkoss.zk.ui.metainfo.impl.ComponentDefinitionImpl.newInstance(ComponentDefinitionImpl.java:296)
org.zkoss.zk.ui.metainfo.ComponentInfo.newInstance(ComponentInfo.java:664)
org.zkoss.zk.ui.impl.AbstractUiFactory.newComponent(AbstractUiFactory.java:90)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:600)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:583)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:527)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:559)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:527)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:494)
org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:375)
org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:296)
org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:229)
org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:166)
javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.16 logs.

How did you encounter this error?
As the package name suggested,
org.zkoss.zss.app.MainWindow should be a sample application utilize zss.jar.

BTW, I'll move the thread to "help", not studio.