seems a little hurdled around..
<?php
$checkboxValues = get_field('field_name'); //Change field_name to your checkbox field
$extraClasses = (is_array($checkboxValues) ? implode(' ', $checkboxValues) : $checkboxValues); //If its an array, implode it! otherwise just use the value
?>
<?php post_class( $extraClasses ); //This goes in your <article> element (or whatever you use) ?>
