View file application/appearance/admin/elements/nav.tpl
<ul>
{if(isset($_SESSION['id']))}
<li><a class="Alternative" href="/account/logout">{echo(getLang('portal_nav_logout'))}</a></li>
{else}
<li><a class="Alternative" href="/account/auth">{echo(getLang('portal_nav_login'))}</a></li>
{/if}
</ul>