Lava (YooThemes)

Under Appearance > Editor, select ‘theme.php (layouts/theme.php)’ on the right.

Find (line 49):

<?php echo $this['widgets']->render('menu'); ?>

1	<?php echo $this['widgets']->render('menu'); ?>

Replace with:

<?php wp_nav_menu( array( 'theme_location' => 'main_menu' ) ); ?>

1	<?php wp_nav_menu( array( 'theme_location' => 'main_menu' ) ); ?>

Max Mega Menu Documentation