from: http://eventespresso.com/topic/checkout-page-and-groupons-promotions/

groupons.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function add_groupon_message_to_promo_input() {
    ?>
    <script>
      jQuery( document ).on('ajaxComplete ready', function() {
        jQuery('div#ee-single-page-checkout-dv input#ee-promotion-code-input')
          .attr('placeholder', 'if you have a voucher, type "GROUPON" here');
        jQuery('div#ee-single-page-checkout-dv #ee-promotion-code-input-lbl em')
           .remove();
        jQuery('div#ee-single-page-checkout-dv #ee-promotion-code-input-lbl')
          .prepend('<div><em>Please Note: If you have a voucher, type "GROUPON" in the field below</em></div>').css('margin-top', '-26px');
      });
    </script>
    <?php

}
add_action( 'AHEE__payment_options__reg_step_start', 'add_groupon_message_to_promo_input' );