Show validation errors on custom elements added using hooks
It should be something like below

function mymodule_civicrm_validateForm($formName, &amp;$fields, &amp;$files, &amp;$form, &amp;$errors) {
  if ('CRM_Event_Form_ManageEvent_EventInfo' == $formName) {
    if ( validation condition fails )
      $errors['custom_element'] = ts('Validation message');
    }
  }
}


HTH