You don’t need a custom walker function. You can alter wp_nav_menu like this:
wp_nav_menu( array(
'theme_location' => 'primary',
'menu_class' => 'flexnav', //Adding the class for FlexNav
'items_wrap' => '<ul data-breakpoint="800" id="%1$s" class="%2$s">%3$s</ul>', // Adding data-breakpoint for FlexNav
));
and proper script init should be:
jQuery(document).ready(function($){
$(".flexnav").flexNav({
});
});
Drop menu button somewhere outside navigation:
<div class="menu-button">Menu</div>
But still does not work. As toscho refered, FlexNav needs a small modification to work in WordPress. Take a look this mod here http://pastebin.com/ZEMQFFsA
UPDATE: Since v.1.0 FlexNav support jQuery noConflict mode