Displayng menu description under main-menu links in Drupal 7
It worked when I used the Main Menu block in header region.

It wasn't working before because the Main Menu was directly being output in the page.tpl.php file by a call to theme('links_system_main_menu'). I followed the instructions on "Descriptions under main menu" at the Fusion Themes forum, and enabled the Main Menu block to get the desired effect.