Mail chimp

if its this one, seems pretty straight forward.
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=632

you have one file to edit before you upload the mod
includes/languages/english/extra_definitions/mailchimp_sidebox_defines.php

just follow the instructions in the read me file to find the info you need to put into that file.

Quote

Pre-Installation Preparation:

0. Read the MailChimp Email Marketing Guide at
http://www.mailchimp.com/resources/email_marketing_guide.phtml

1. Sign up for mailchimp at http://www.mailchimp.com

2. Create a list for your newsletter (press the Lists tab
at the top of the page, then "Create New List"). In your Sign-up form, I suggest
only having email address (if you want more fields, you'll
have to modify this contribution).

3. Press the "Forms" link for your new list.
This will
take you to a screen which will show you your signup form.
Press the "create embed code for small form" link.

It generates code in a box on the left hand side of the screen under
the label copy/paste onto your site.

Search for the string "list-manage.com".

You will find something that looks like this:

form action="http://YourSite.us1.list-manage.com/subscribe/post?u=d0bcc6a7b231472e0ac481ae0&id=56d217bcfc" method="post" ....

Note the the values of "u" and "id" - you will be
embedding them in the language file during the installation process.
The & is not part of the u value; in this case,

u is d0bcc6a7b231472e0ac481ae0
id is 56d217bcfc

Note also the value of the URL before the arguments; in this case, it is

http://YourSite.us1.list-manage.com/subscribe/post

4. At the top of the Mailchimp page is a dropdown called "Account."
Select "api keys & info" and add an api key to your account.
Note the value; you will be embedding
it in the language file during the installation process.

5. Take the time to test and customize MailChimp before
adding it to your site.

--------------------------------------------------

Installation Instructions:
0. Back up everything! Try this in a test environment prior to installing
it on a live shop.

1. Copy the contents of the folder you have unzipped to
the root directory of your shop.

2. Edit the file
includes/languages/english/extra_definitions/mailchimp_sidebox_defines.php
set the BOX_MAILCHIMP_NEWSLETTER_ID to the "id" value you noted
in step 3 of the pre-installation instructions.
set the BOX_MAILCHIMP_NEWSLETTER_U to the "u" value you noted
in step 3 of the pre-installation instructions.
Set BOX_MAILCHIMP_NEWSLETTER_URL to the value you noted in step 3 of
the pre-installation instructions.
Set BOX_MAILCHIMP_APIKEY to the value you noted in step 4 of the
pre-installation instructions.

3. In Admin->Tools->Layout Boxes Controller, turn on the mailchimp
sidebox.

4. Changes to your Zen Cart configuration:
a) In Configuration->Customer Details, set Show Newsletter Checkbox to 1.
b) In Configuration->Email Options, set Display 'Newsletter Unsubscribe'
Link to false.
(To subscribe, people use the MailChimp sidebox; to unsubscribe, they
use the link at the bottom of the newsletter.)

5. If you do not yet have an
includes/modules/YOUR_TEMPLATE/create_account.php,

copy from includes/modules/create_account.php to
includes/modules/YOUR_TEMPLATE/create_account.php

Look for the line:
$zco_notifier->notify('NOTIFY_LOGIN_SUCCESS_VIA_CREATE_ACCOUNT');

right below it, add the code:

if ((int)$newsletter == 1) {
mailchimp_add($email_address, $email_format);
}