an error has occured while executing my project...

one file is RegistrationForm.jsp i places it in c:/tomcat6.0/webapps/
SWF/CAP/
error::

org.apache.jasper.JasperException: Unable to compile class for JSP:

An error occurred at line: 16 in the jsp file: /CAP/
RegistrationForm.jsp

nl.captcha.servlet.Constants.SIMPLE_CAPCHA_SESSION_KEY cannot be
resolved to a type
13: <%
14: try
15: {
16: String c = (String)
session.getAttribute(nl.captcha.servlet.Constants.SIMPLE_CAPCHA_SESSION_KEY);
17: String parm = (String) request.getParameter("captchafield");
18:
19: //out.println(parm + " ? " + c + ":");

the file RegistrationForm.jsp
 <%
            String c = (String)
session.getAttribute(nl.captcha.servlet.Constants.SIMPLE_CAPCHA_SESSION_KEY);
            String parm = (String) request.getParameter("captchafield");

            //out.println(parm + " ? " + c + ":");

            if (c != null && parm != null) {
                if (c.equals(parm)) {
                    out.println("<b>Form Registered Successfully</b>");
                    response.sendRedirect("index.jsp?todo='Registration Completed
Successfully...'");
                } else {
                    out.println("<b>Please Re-Enter the Code Correctly and Then
Submit</b>");
                }
            }
and the servlet code is::

package nl.captcha.servlet;

/**
 * @author Administrator
 *
 * To change the template for this generated type comment go to
 * Window&gt;Preferences&gt;Java&gt;Code Generation&gt;Code and
Comments
 */
public class Constants {

    public final static String SIMPLE_CAPCHA_SESSION_KEY =
"SIMPLE_CAPCHA_SESSION_KEY";
    public final static String SIMPLE_CAPCHA_TEXTPRODUCER =
"cap.text.producer";
    public final static String SIMPLE_CAPTCHA_BG_IMP = "cap.bg.imp";
    public final static String SIMPLE_CAPTCHA_BOX = "cap.border";
    public final static String SIMPLE_CAPTCHA_BOX_C = "cap.border.c";
}

actually this might be simple problem but i cannot understand the
problem.. can u please help me....