Hi there!
It is not so easy
The developers of docman have to put the following code in there template:
<div class="dm_description" style="text-align:right;">
<?php
global $option, $mosConfig_absolute_path;
require_once("$mosConfig_absolute_path/administrator/components/com_comment/plugin/$option/josc_com_docman_item.php");
?>
</div>
Since this code is not there you have put it yourself there. Open the following file:
components/com_docman/themes/default/templates/documents/document.tpl.php
and add the above lines in the end of it.
This should make the comments to appear!
Greetings!
Daniel