I guess you want a menu that collapses and becomes vertical at small screen sizes. Right now you have a menu that stays horizontal and shrinks to oblivion on small screens.
Set a breakpoint a little before the menu starts looking squashed. Then in Design mode, the top rightmost option says Layout. Open that and select the version where it says Recommended for small screens. Then it will collapse.
Another thing you need to do, is on the web pages where this menu is to be used, add a metatag for viewport:
<meta name="viewport" content="width=device-width, initial-scale=1">
Add it right at the top below the meta tag for the charset. In the programme MenuBuilder the menu will be responsive even without this line, but it is necessary on web pages.