how to open drupal login and signup form on modal form.
You should include the necessary javascript in your theme .info file or template or module with drupal_add_js() , then you should use the code defined in the link you provide to target the element that you want to apply the functionality too. This elememnt may be generated by a module that specifies a theme function and uses a template (tpl.php) file in which you would put your markup.