This would require custom code to accomplish.
you could validate the checkboxes using the following code:
<?php
if($_SERVER['REQUEST_METHOD'] == "POST") {
if((!isset($_POST['field1']) || $_POST['field1'] == "") && (!isset($_POST['field2']) || $_POST['field2'] == "") && (!isset($_POST['field3']) || $_POST['field3'] == "") && (!isset($_POST['field4']) || $_POST['field4'] == "") && (!isset($_POST['field5']) || $_POST['field5'] == "")) {
@session_start();
$_SESSION['failed'] = "true";
header("Location: ". $_SERVER['PHP_SELF']);
exit;
}
}
?>
this assumes the checkboxes are named "field1", "field2", "field3", "field4", and "field5"
then use this code to show an error message;
<?php @session_start(); if(isset($_SESSION['failed']) && $_SESSION['failed'] == "true") { ?>At least one box must be checked<?php unset($_SESSION['failed']); } ?>