You could do this with some php in the MTEntries part of your template:
CODE
<?
$mtentrydate = "<MTEntryDate format="%m%d%Y%H%I%S">";
$entrymonth = substr($mtentrydate, 0,2);
$entryday = substr($mtentrydate, 2,2);
$entryyear = substr($mtentrydate, 4,4);
$entryhour = substr($mtentrydate, 8,2);
$entryminute = substr($mtentrydate, 10,2);
$entrysecond = substr($mtentrydate, 12,2);
$entry_date = mktime($entryhour, $entryminute, $entrysecond, $entrymonth, $entryday, $entryyear);
$current_date = strtotime ("now");
$diff = $current_date - $entry_date;
$days = ($diff/86400);
$num_days = intval($days);
$hours = (($days - $num_days) * 24);
$num_hours = intval($hours);
$mins = (($hours - $num_hours) * 60);
$num_mins = intval($mins);
$secs = (($mins - $num_mins) * 60);
$num_secs = intval($secs);
echo "Posted $num_days days, $num_hours hours, $num_mins minutes, $num_secs seconds ago. <br />";
?>