How to Add Dynamic Menus to a WordPress Theme

functions.php

For single menu:

function wordpress_menu() {
    if (function_exists('register_nav_menu')) {
        register_nav_menu( 'main-menu', __( 'Main Menu') );
    }
}
add_action('init', 'wordpress_menu');

Call funtion:
<?php wp_nav_menu( array( 'theme_location' => 'main-menu') ); ?>

Example:
<div class="header_menu">
<?php wp_nav_menu( array( 'theme_location' => 'main-menu') ); ?>
</div>

For multiple menus:

function register_my_menus() {
if (function_exists('register_nav_menu')) {
  register_nav_menus(
array(
  'header-menu' => __( 'Header Menu' ),
 'extra-menu' => __( 'Extra Menu' ),
  'sidebar-menu' => __( 'Sidebar Menu' ),
  'footer-menu' => __( 'Footer Menu' )
)
);
}
}
add_action( 'init', 'register_my_menus' );

Call

<?php wp_nav_menu( array( 'theme_location' => 'header-menu') ); ?>
or
<?php wp_nav_menu( array( 'theme_location' => 'extra-menu') ); ?>
or
<?php wp_nav_menu( array( 'theme_location' => 'footer-menu') ); ?>

If yo want to use direct CSS then put a class:
<?php wp_nav_menu( array( 'theme_location' => 'extra-menu', 'container_class' => 'my_extra_menu_class' ) );?>

Another Example

functions.php:
add_action('init', 'dji_register_menu');
function dji_register_menu() {
if (function_exists('register_nav_menu')) {
register_nav_menu( 'freedom-main-menu', __( 'Main Menu', 'themename' ) );
}
}
register_nav_menu( 'footer-menu', __( 'Footer Menu', 'themename' ) );
register_nav_menu( 'sidebar-menu', __( 'Sidebar Menu', 'themename' ) );

function dji_default_menu() {
echo '<ul id="nav">';
if ('page' != get_option('show_on_front')) {
echo '<li><a href="'. home_url() . '/">Home</a></li>';
}
wp_list_pages('title_li=');
echo '</ul>';
}

Usage or Call:
<?php
if (function_exists('wp_nav_menu')) {
wp_nav_menu(array('theme_location' => 'freedom-main-menu', 'menu_id' => 'dropmenu', 'fallback_cb' => 'dji_default_menu'));
}
else {
dji_default_menu();
}
?>