1) You can't just throw php code in your HTML.
Whatever you are trying to, put that php code between {php} and {/php} when you want to use php code in tpl files
so smarty knows that you want to interpret it as php.
2) you can't use {$percent} if you haven't passed that variable to smarty. So give that variable to smarty in the php.
To sum up, do something like this:
Code:
<tr>
<td><strong>{#PLIGG_Visual_User_Profile_Total_Votes#}:</strong></td>
{php}
$percent = round($user_total_votes/$queued), 2);
$main_smarty->assign('percent', '$percent');
{/php}
<td>{$user_total_votes} ({$percent} %)</td>
</tr>