wp_editor() outputs the textarea(html codes), so, maybe you dont need to use that in functions.php, but use inside the page's source. (for example, in your plugin.php:

<div class="blabla>
  <form action="" method="POST">
  <?php wp_editor( 'Hi,its content' , 'desired_id_of_textarea', $settings = array('textarea_name'=>'your_desired_name_for_$POST') ); ?> 
  <input type="submit">
  </form>
</div>

wp_editor() outputs the textarea(html codes), so, maybe you dont need to use that in functions.php, but use inside the page's source. (for example, in your plugin.php:

<div class="blabla>
  <form action="" method="POST">
  <?php wp_editor( 'Hi,its content' , 'desired_id_of_textarea', $settings = array('textarea_name'=>'your_desired_name_for_$POST') ); ?> 
  <input type="submit">
  </form>
</div>

note: it may wont work for add_option function..