Hello Komoridako,
 
Please let me know what the theme you are using? For import megamenu json, please try to update the latest version of module or setup if via ves_tempcp > datasample. The live menu editor you should add root menu item before use it (and the live menu editor we just allow manage sub menu items by widgets) Please view video guide: https://youtu.be/vlEqUfJigOA