show the meta values to visitors that collected via coment form

In the link you have provided it shows how to get the value of the “collected” meta using

$twitter = get_comment_meta($comment_ID,'_twitter',true);

so all that is left for you to do is edit your comments loop and add the next few lines in it.

$extra_field = get_comment_meta($comment_ID,'_extra_field',true);
echo $extra_field;

Hope this Helps