Trouble inserting custom menu

Hey all, So I used this guide: https://codex.www.ads-software.com/Navigation_Menus so that my theme would register a custom menu. What I am having trouble with is where to put the

<?php wp_nav_menu( array( ‘theme_location’ => ‘header-menu’ ) ); ?>

So that my menu bar is displayed. My site is:
https://dev.progressivemediaconcepts.com/

I want it to replace the current menu that is listed, but the menusm.css seems to be what creates the layout of the nav bar currently.

Anyone know how to keep the format it currently has, but use the custom menu?