Access $Sesssion from helper in cakephp

hello folks.

a cakePHP newbie here....

I have created a custom helper.

I need to get a session value in this helper and i need to get some data from a table.

How i can make these things possible.

I have tried

var $helper=array('Session');

but then also when i use

$this->Session->read('userid');

it returns error

Undefined property: CustomHelper::$Session

here is the helper in detail

<?php 
class CssMenuHelper extends Helper{

    var $helpers = array('Html','javascript','Session');

    function createMenu(){

        $gid=$this->Session->read('Auth.Login.group_id');

       }
   }
  ?>