Saving multiple checkbox meta data WordPress

Code works!

I have two custom post types: Events and Artists. I’m using WPAlchemy MetaBox PHP class , trying to create a metabox with a series of dynamically created checkboxes (i.e. there will be a checkbox for each Artists post) in the Events post editor that will allow me to select which Artists are appearing at an Event.

Any help or insight much appreciated! Thanks!

This code works fine to display the checkboxes (from checkbox_meta.php):

<div class="my_meta_control">

    <label>Group checkbox test #2</label><br/>

        <?php
        global $post;
        $artists = get_posts('post_type=artists');
        foreach($artists as $artist) :
        setup_postdata($artist);
        $slug = $artist->post_name;
        ?>

        <?php $mb->the_field('cb_ex2', WPALCHEMY_FIELD_HINT_CHECKBOX_MULTI); ?>

        <input type="checkbox" name="<?php $mb->the_name(); ?>" value="<?php echo $slug; ?>"<?php $mb->the_checkbox_state($slug); ?>/><?php echo $artist->post_title; ?><br/>

    <?php endforeach; ?>

    <input type="submit" class="button-primary" name="save" value="Save">

</div>

This code from functions.php:

include_once 'assets/functions/MetaBox.php';
if (is_admin()) wp_enqueue_style('custom_meta_css', 'wp-content/themes/bam/assets/css/meta.css');

define('THEMEASSETS', STYLESHEETPATH . '/assets');

$custom_metabox = new WPAlchemy_MetaBox(array
(
    'id' => '_custom_meta',
    'title' => 'My Custom Meta',
    'types' => array('sp_events'),
    'template' => THEMEASSETS . '/functions/checkbox_meta.php'
));