css - main_menu variable is always empty in drupal theme Zen

Question

I am new to drupal theming and trying to edit the menu for a sub theme with base theme as Zen. But when I tried to add class and div to the main menu, I saw its not changing. Later when I print_r($main_menu) in page.tpl.php in my subtheme directory, it is showing empty results ! But the same print_r($main_menu) shows the main menu array in Bartik theme's page.tpl.php ! How can I create the main_menu variable in my theme ?