The popular posts plugin was broken, but it’s fixed now. Update your template so it looks like this if you want to use it:
{popularposts}
{if $pposts != ''}
<br />Most Popular Posts<br /><br />
{foreach from=$pposts key=key item=details}
<nobr><a href="{$details.url}" title='{$details.title}'>{$details.title|truncate:30:"..."}</a>: {$key}</nobr><br />
{/foreach}
{/if}

