WordPress.org

Upload contents of zip archive to your wp-content/plugins/ folder and activate plugin from WP admin area.

Then, open up your template file (any of them, where you want your greeting to appear, I use comments.php) and insert following code:

<?php
if (function_exists('wp_vokativ')) :
?>
<p class="vokativ"><?php wp_vokativ(); ?></p>
<?php
endif;
?>

This basically checks if wp_vokativ() function is defined (plugin is loaded) and if so, it calls the function.