I thought it beneficial to note this support page on integrating the great Flex Nav script with WordPress.

You don’t need a custom walker function. You can alter wp_nav_menu like this:

<pre><code>wp_nav_menu( array( 
    'theme_location' => 'primary',
    'menu_class' => 'flexnav', //Adding the class for FlexNav
    'items_wrap' => '<ul data-breakpoint="800" id="%1$s">%3$s</ul>', // Adding data-breakpoint for FlexNav
    ));</code></pre>

and proper script init should be:

<pre><code>jQuery(document).ready(function($){
   $(".flexnav").flexNav({
});
});</code></pre>