This plugin works wonderful with the default theme but the Twitter field does not show with my default theme. I tested this on a clean WordPress installation.
In my comments.php I identified:
<fieldset>
<label for="author">Name <?php if ($req) echo '<span class="small-text">(required)</span>'; ?></label>
<input type="text" name="author" id="author" value="<?php echo $comment_author; ?>" size="50" />
</fieldset>
Can I just add a similar <fieldset> to make the Twitter field appear? What should it be exactly?
http://wordpress.org/extend/plugins/twitter-avatar-reloaded/