[Plugin: WP Simple Galleries] Add to custom post type?

How to add this functionality to custom post types?

in the file wp-simple-gallery.php from line 70 tp 77 it seems there’s the function to add the meta box to posts and pages

public function add_meta_boxes() {
        add_meta_box(
                'wpsimplegallery', __('WP Simple Gallery', 'wpsimplegallery'), array(&$this, 'inner_custom_box'), 'post'
        );
        add_meta_box(
                'wpsimplegallery', __('WP Simple Gallery', 'wpsimplegallery'), array(&$this, 'inner_custom_box'), 'page'
        );
    }

but if i change to:

add_meta_box(
                'wpsimplegallery', __('WP Simple Gallery', 'wpsimplegallery'), array(&$this, 'inner_custom_box'), 'MY-CUSTOM-POST-TYPE-NAME'

No meta box appears in my custom post type.
Any sugestion?
Thanks in advance

http://wordpress.org/extend/plugins/wp-simple-galleries/