In the keyword criteria for the checkbox, make sure to set the OR separator to ", " (a comma followed by a space).
with a multiple checkbox, the selections are sent as an array, the array needs to be converted to a comma separated list.
add the following code at line 1 for this:
<?php
if($_SERVER["REQUEST_METHOD"] == "POST") {
foreach($_POST as $key => $val) {
if(is_array($_POST[$key])) $_POST[$key] = implode(", ", $_POST[$key]);
}
}
?>