java.lang.NoClassDefFoundError on initialisation

javax.servlet.ServletException
   org.apache.jasper.runtime.PageContextImpl.doHandlePageException(PageContextImpl.java:846)
   org.apache.jasper.runtime.PageContextImpl.handlePageException(PageContextImpl.java:779)
   org.apache.jsp.jsp.urban.urbanMagEditor_jsp._jspService(urbanMagEditor_jsp.java:69)
   org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
   javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
   org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
   org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
   org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
   javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

cause mère

java.lang.NoClassDefFoundError
   net.fckeditor.FCKeditor.<init>(FCKeditor.java:51)
   net.fckeditor.tags.EditorTag.doStartTag(EditorTag.java:125)
   org.apache.jsp.jsp.urban.urbanMagEditor_jsp._jspx_meth_FCK_editor_0(urbanMagEditor_jsp.java:85)
   org.apache.jsp.jsp.urban.urbanMagEditor_jsp._jspService(urbanMagEditor_jsp.java:60)
   org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
   javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
   org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
   org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
   org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
   javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

  <%@ taglib uri="/FCKeditor.tld" prefix="FCK" %>

  <FCK:editor instanceName="EditorDefault" >
  </FCK:editor>