Dynamically Highlighted Navigation

Hi! I am new to WP. I have found problem in highlighting current page Navigation.
In my navigation part I have used following code:

<div id=”menu”>
<ul id=”nav”>
<?php wp_nav_menu(array(‘menu’=>’Main Nav Menu’)); ?>

</div>

<!– end menu –>

In php function file I have added following code
function.php

//Declare main menus

if (function_exists('register_nav_menus')) {
    register_nav_menus(
            array(
                'main_nave' => 'Main Nav Menu'
    ));
}

I have defined menu dynamically.
Thank you for you help.