I am confused either to create a new plugin or make some changes to the source code to add a menu option.
What should i do to add a main menu option in Mantis BT?
If all you need is a menu item pointing to a static link, the easiest way to do this is via configuration; add the following code to your config_inc.php file:
$g_main_menu_custom_options = array(
array( 'Link text', ANYONE, 'http://example.com' ),
);
Please refer to the Admin guide for details.