I believve that you want to add a "reply" link to the sub-comments.
Open /templates/yourtemplate/comment_show.tpl, and look for the line <div class="comment-info">
Modify the code as follows (modifications and additions are in red) :
Code:
<div class="comment-info">
{if $comment_parent >= 0 && $current_userid neq 0}
<a href = "javascript://" {if $comment_parent eq 0}onClick="var replydisplay=document.getElementById('reply-{$comment_id}').style.display ? '' : 'none'; document.getElementById('reply-{$comment_id}').style.display = replydisplay;"{else}onClick="var replydisplay=document.getElementById('reply-{$comment_parent}').style.display ? '' : 'none'; document.getElementById('reply-{$comment_parent}').style.display = replydisplay;"{/if}>{#PLIGG_Visual_Comment_Reply#}</a>
<div id="reply-{$comment_id}" style="display:none;" align="left">
<fieldset><legend>{#PLIGG_Visual_Comment_Send#}</legend>
<form action="" method="POST" id="thisform" style="display:inline;">
<label>{#PLIGG_Visual_Comment_NoHTML#}</label>
<textarea name="reply_comment_content-{$comment_id}" id="reply_comment_content-{$comment_id}" rows="3" cols="55"/>{$TheComment}</textarea><br/>
<!-- {if $Spell_Checker eq 1}<input type="button" name="spelling" value="{#PLIGG_Visual_Check_Spelling#}" onClick="openSpellChecker('reply_comment_content-{$comment_id}');" class="submit"/>{/if} -->
<input type="submit" name="submit" value="{#PLIGG_Visual_Comment_Submit#}" class="submit" />
<input type="hidden" name="process" value="newcomment" />
<input type="hidden" name="randkey" value="{php}echo rand(1000000,100000000);{/php}" />
<input type="hidden" name="link_id" value="{$comment_link}" />
<input type="hidden" name="comment_parent_id" value="{$comment_id}" />
<input type="hidden" name="user_id" value="{$current_userid}" />
</form>
</fieldset>
</div>
{/if} It works.