Q:

add a menu item to navigation menu to link to external url

I followed this tutorial to add a menu item to navigation menu to link to external site:
http://www.natenewz.com/2009/10/07/hardlink-wordpress-navigation/
I copied the snippet of code to my theme's header.php
I created a page  with the name having the text that I want to appear in the navigation
I added a custom field to the page called redirect and placed the link url into the value field.
And then tested and the link never appears in the menu.

A:

Are you trying to navigate to an external page or redirect? (Also, bear in mind that was posted almost 3 years ago...in websites and technology in general, that's an eternity!)
You could always just add a custom link to the WordPress menu under Appearance->Menus. Just make sure you add support in the functions.php file.
http://codex.wordpress.org/Appearance_Menus_Screen
And here for adding theme support:
http://codex.wordpress.org/Navigation_Menus
This is how you would add functional support:
//setup the dynamic navigation
if (function_exists(register_nav_menus)){
    register_nav_menus(
                 array(
                "main_nav" => "Main Navigation Menu"
                      )
                );
 }

Then go to the admin panel->Appearance->Menus, and add your external link there.
Then in your template, you would call it like so:
<?php wp_nav_menu(array('menu' => 'Main Nav Menu')); ?>

This is, ofcourse, what you were looking for

