Adding items to a WordPress menu dynamically. http://www.datafeedr.com/?p=1639

functions.php

<?php

/**
 * Adds a link to the Primary Menu based on the 
 * user's logged in status.  This is to be placed
 * in your theme's functions.php file or in your 
 * own plugin file.
 * 
 * This example works for the WordPress Twenty-
 * Twelve theme.
 */
function twentytwelve_wp_nav_menu_items($items, $args) {

	// Make sure this is the Primary Menu.
	// You may need to modify this condition
	// depending on your theme.
	if ($args->theme_location == 'primary') {
	
		// CSS class to use for <li> item.
		$class = 'menu-item';

		if (is_user_logged_in()) {
			// User is logged in, link to welcome page.
			$extra = '
			<li id="menu-item-logged-in-user" class="'.$class.'">
				<a href="/welcome">
					'.__('Welcome').', '.wp_get_current_user()->user_login.'!
				</a>
			</li>
			';
		} else {
			// User is guest, link to login page.
			$extra = '
			<li id="menu-item-logged-out-user" class="'.$class.'">
				<a href="/wp-login.php">
					'.__('Log in').'
				</a>
			</li>
			';
		}

		// Add extra link to existing menu.
		$items = $items . $extra; 
	}
	
	// Return menu items.
	return $items;
	
}

// Hook into wp_nav_menu_items.
add_filter( 'wp_nav_menu_items', 'twentytwelve_wp_nav_menu_items', 10, 2 );

?>