Trouble implementing URL based authorization

I am having trouble implementing URL based authorization where in i am trying to restrict the access to a particular folder
in a web application (say Admin/). I created a web.config under the Admin folder and following is the snapshot of the web.config
code:

        <system.web>
            <authorization>              
                <allow roles="Power Users"/>
                <allow roles="Guests"/>
            </authorization>
        </system.web>

when i run this secured appli ...