1. First go to Admin > Manage Categories > Edit Category, In textarea Custom Code, enter meta description....
2. themes/THEME/categories.php
Replace this code:
<meta name="keywords" content="{$keywords}" />
<meta name="description" content="{$metadesc}" />
<link href="themes/{$themes}/styles/images/favicon.ico" type="image/x-icon" rel="shortcut icon" />
<link rel="stylesheet" type="text/css" href="themes/{$themes}/styles/categories.css" />
<link rel="alternate" type="application/atom+xml" title="{$sitetitle} - RSS" href="{$sitepath}/rss.php" />
<title>{$sitetitle} {if isset($smarty.get.next)}[{$smarty.get.next}]{else}{/if}</title>
with this code:
<link href="themes/{$themes}/styles/images/favicon.ico" type="image/x-icon" rel="shortcut icon" />
<link rel="stylesheet" type="text/css" href="themes/{$themes}/styles/categories.css" />
<link rel="alternate" type="application/atom+xml" title="{$sitetitle} - RSS" href="{$sitepath}/rss.php" />
<meta name="keywords" content="{$keywords}" />
{foreach from=$categori item=metalink}
{if $metalink.catid eq $smarty.get.id}
{if $metalink.cuscat eq true}<meta name="description" content="{$metalink.cuscat}" />{else}<meta name="description" content="{$metadesc}" />{/if}
<title>{$metalink.name|stripslashes} - {$sitetitle} {if isset($smarty.get.next)}[{$smarty.get.next}]{else}{/if}</title>
{/if}
{/foreach}