I've got this code under tags

<h3><a href="#"></a></h3>
<div>
<table class="admintable">
<tr>
<td class="admintable">
<td align="right" class="key">
<?php echo JText::_('Resumen en 150 caracteres'); ?>
</td>
<td>
<textarea maxlength="150" name="metadesc" rows="5" cols="20"><?php echo $this->row->metadesc; ?></textarea>
</td>
</tr>
<tr>

But I don't see the metadesc on backend, and also I don't see it in firefox analysis on the head.

Help, please.

Sorry about my english.

Where exactly are you getting this code? This code is probably not part of K2, check for any third-party plugins. You can always set the meta description in the item edit form ( at the sidebar under the "Metadata information" ).