Update post-menu-widget metabox description

custom-field-types/associate-wp-menu-field.php

@@ -100,6 +100,9 @@ function post_menu_edit_or_create_link( $args, $field ) {
 
 		?>
 		<a href="<?php echo esc_url( $url ); ?>"><?php echo $link_title; ?></a>
+
+		<p class="cmb2-metabox-description explain-widget-necessary"><strong>Note:</strong> In order for this menu to display, you will need to ensure the "Associated Post Menu" widget is placed in the <a href="<?php echo admin_url( 'widgets.php' ); ?>">widget area</a> for this template.</p>
+
 		<?php if ( ! $script_added ) : ?>
 			<script type="text/javascript">
 				if ( '#<?php echo $this->metabox_id; ?>' === window.location.hash ) {
@@ -111,6 +114,11 @@ function post_menu_edit_or_create_link( $args, $field ) {
 				.post-menu-box-highlighted {
 					box-shadow: 0 0 10px 5px rgba(226, 73, 73, 0.28);
 				}
+				#side-sortables #<?php echo $this->metabox_id; ?> .explain-widget-necessary {
+					padding-bottom: 0;
+					padding-top: 10px;
+					margin-bottom: -15px;
+				}
 			</style>
 		<?php endif;