set enableSessionState to true

This is what is shown in my the design mode of visual studio for a page of my web site project:
"menuPageSession state can only be used when enableSessionState is set to true,
 either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in
 the <configuration>\<system.web>\<httpModules> section in the application configuration."
So this the line i add to my web.config file:
<pages buffer="true" enableSessionState="true"></pages>
    <httpModules>
      <add type="System.Web.SessionState.SessionStateModule" name="Session"/>
    </httpModules>
Under the "system.web"  tag, but the problem still persist