Resolved:
Edit mf_posttype.php on line 92 and add key "div_class" in array "quantity"
'quantity' => array(
'id' => 'posttype-quantity',
'type' => 'checkbox',
'label' => __( 'Quantity', $mf_domain ),
'name' => 'mf_posttype[core][quantity]',
'value' => 0,
'description' => __( 'mark true if you want your post type only has one element.', $mf_domain ),
'div_class' => ''
)