Help with code for putting Pages and Categories in top navigation buttons

I searched the forums and found this solution several times, but I do not know Php, and so do not know exactly where to put the code or how to write it correctly.

Here is the code that exists in my header.php

<?php
                   if(get_option('fusion_topnav')=='categories') { echo preg_replace('@\<li([^>]*)>\<a([^>]*)>(.*?)\<\/a>@i', '<li$1><a$2><span><span>$3</span></span></a>', wp_list_categories('show_count=0&echo=0&title_li='));  }
                  { echo preg_replace('@\<li([^>]*)>\<a([^>]*)>(.*?)\<\/a>@i', '<li$1><a$2><span><span>$3</span></span></a>', wp_list_pages('echo=0&orderby=name&title_li=&')); }
                  ?>

Here is what was suggested:

Add <?php wp_list_categories(); ?>, either to your theme sidebar.php, or into header.php
Add it right after <?php wp_list_pages(); ?> in header.php .The menu is showing both pages and categories

How do I write that, and where do I put it?

Thank you in advance.