struts2-ssl-plugin problem

[Louis Voo]

Hi,

I download this plug-in and put into my /lib, and in my struts.xml has this

<package name="default" extends="struts-default" namespace="/">
        <default-interceptor-ref name="secureStack" />
    </package>
  
when I start the tomcat, it throw error :

Caused by: Unable to find interceptor class referenced by ref-name secureStack 

What else I need to configure to use this plug-in?

Thanks
Louis