[Nicko]

window.onload = function() {
  var box = document.getElementById('secuCodeMc3');
  box.onchange = function() {
    var button = document.getElementById('mchatBtn')
    if (this.checked) {
      button.style.display = 'none';
    } else {
      button.style.display = '';
    }
  }
}

// Или через jQuery
$(function() {
  $("#secuCodeMc3").change(function() {
    var button = $("#mchatBtn");
    if (this.checked) {
      button.hide();
    } else {
      button.show();
    }
  });
});