If the above is not the issue – Then where is the description input?

Excerpt from the plugin FAQ

Therefore, as of version 1.6 I am modifying my admin nav menu Walker to only adding the following lines (right after the description input):

<?php
// Place this in your admin nav menu Walker
do_action( 'wp_nav_menu_item_custom_fields', $item_id, $item, $depth, $args );
// end added section
?>

Ask your conflicting plugin/theme’s author to add this code to his plugin or theme and our plugins will become compatible.