Display meta box on post

Hi,

First off, this looks exactly like the plugin I need. The documentation is great. However, I don’t understand how I’m supposed to display the content of the meta box on my post, or I don’t know how to insert the code properly.

I’ve managed to create a meta box in a file called specifications.php.
In this PHP file, I have defined one field called “length”.
In my post, I gave ‘1m’ as a value for that field.

In the documentation, it says that I need to insert that piece of code to display the field on the post :
rwmb_meta( $key, $args = array(), $post_id = null );

Here is the meta part of the code of my post PHP file (with the rwmb_meta code added in the post_info_single part):

<div class="post_meta">
                            <div class="post_gallery">
                                <?php
                                  get_post_mainimg($post->ID, 220, 580);
                                ?>
                                <div class="clear"></div>
                            </div>

                            <div class="post_info_single">
                                <?php echo_post_meta_single_info(); ?>
                                <?php echo rwmb_meta('length'); ?>
                            </div><!-- END div.post_info_single -->
                        </div><!-- END div.post_meta -->

Here is what the post looks like: https://imageshack.us/f/203/singlepost.png/

Why doesn’t the field display in the meta info part (right column) ? Did I forget to add something somewhere ? Should I add some code in the functions.php file, besides the call to include specifications.php ?

How can I display the “Length” field on my post ?

Thanks for the help.