Help! Can't make it work

Hi!

I've just bought the extension.
Installed through the plug in install.
I'm working on a child theme that inherits twentyfifteens.

In my functions.php I'm adding the following:

add_filter( 'rwmb_meta_boxes', 'hslt_register_meta_boxes' );

function hslt_register_meta_boxes( $meta_boxes )
{
    $prefix = 'hslt_';

    // 1st meta box
    $meta_boxes[] = array(
        'id'       => 'ImgText',
        'title'    => 'Testo Immagine',
        'pages'    => array( 'page' ),
        'context'  => 'normal',
        'priority' => 'high',

        'exclude' => array(
                'relation' => 'OR',
                'ID' => array( 58 )
            ),

        'fields' => array(
            array(
                'name'  => 'Testo Immagine',
                'desc'  => 'Testo che compare al di sotto delle immagine nei template di primo e secondo livello',
                'id'    => $prefix . 'testoImg',
                'type'  => 'wysiwyg',
                'std'   => 'Anh Tran',
                'class' => 'custom-class',
                'clone' => true
            ),
        )
    );

    return $meta_boxes;
}

The problem is that I can see the custom metabox in the page editor and in the client side version.

What I'm missing? Did I jumped some steps?

My final goal is to create metaboxes visible only on specific pages.

This is how I print the metaboxes value in the fornt end:

<?php
            $textboxes = rwmb_meta( 'hslt_testoImg' );

            if (is_array($textboxes))
            {
                foreach ($textboxes as $value)
                {
                    echo $value;
                }
            } else {
              echo "Qualcosa è andato storto. Contatta l'amministratore.";
            }
          ?>

Help please!

thank you and have a good day.