Hi fragallegos,
Thanks for getting in touch.
Are you trying to add search boxes through the theme customiser interface? This plugin does not yet support that as it was still being developed until very recently. In order to add search boxes, you need to use the “old” interface for adding menus, which is located at https://your.domain/wp-admin/nav-menus.php.
If you still can’t add search boxes to the menu, this is likely due to an annoying default behaviour of WP, which is to hide many options on first access to nav-menus.php. To overcome this, you need to open the screen options tab (at the top of the screen) and tick the appropriate checkbox.
If the search box tab still isn’t showing, it is likely there is a conflict with another plugin or theme (and that other party’s code is pretty much hijacking the entire nav-menus interface).
Hope this helps.
If this solves your issue, please mark this topic as resolved. Otherwise, if you could provide details of your workflow for editing menus, I’ll try to help some more.
Cheers,
Joe