Hi Istvan,

ensure that the menu is assigned to the correct position to be displayed and rendered as a mobile menu by Responsivizer.

The main menu module must be assigned to the position responsivizer_mainmenu or responsivizer_menu_top.
You find all various menu configuraiton in the pdf docs at the paragraph 'Mobile menu positions and management'

If your desktop template is not using a Joomla module to render the menu, just create a module instance for the main menu and assign it for example to the position responsivizer_mainmenu

Best regards,
John