Hi Anne
I'm assuming you are using the 'Ask a Question' plugin - https://www.zen-cart.com/downloads.php?do=file&id=109
If so, it should be fairly easy to add it to that.
Essentially you need too look at the instruction for pre-1.5.1 installation

In /includes/modules/ask_a_question/header.php just after line 41 add:
Code:
$zco_notifier->notify('NOTIFY_ASK_A_QUESTION_CAPTCHA_CHECK');
and change the line below it from
Code:
if ($zc_validate_email and !empty($enquiry) and !empty($name)) {
to
Code:
if ($zc_validate_email and !empty($enquiry) and !empty($name) and $error == false) {
in /includes/classes/observers/class.google_recaptcha.php insert this just below line number 23
Code:
$pages_to_check[] =  'NOTIFY_ASK_A_QUESTION_CAPTCHA_CHECK';
in your tpl_ask_a_question_default.php file (probably just under the textarea, so line 80) add
Code:
<?php echo recaptcha_get_html(); ?>
That should do it :)