It should be something like below
<br><br>
<code>
function mymodule_civicrm_validateForm($formName, &$fields, &$files, &$form, &$errors) {
  if ('CRM_Event_Form_ManageEvent_EventInfo' == $formName) {
    if ( validation condition fails )
      $errors['custom_element'] = ts('Validation message');
    }
  }
}
</code>
<br>
HTH<br>
Pradeep