The MailChimp add-on is a premium add-on for iThemes Exchange.
To use the MailChimp add on, you'll first need to create an account and a list at MailChimp.com.
On your site, you'll install the Exchange MailChimp add-on as you would any other plugin on your site. To enable/disable the MailChimp add-on, you'll go into Exchange > Add-ons.
Once installed and enabled, you'll go to the settings (the gear symbol next to the Enabled button on the Add-ons page).
You'll need to go into your MailChimp settings and then Extras > API Keys to create the key you'll use on this site.
There, you'll find a button to create an API Key then copy that key.
Back in your MailChimp add-on settings in Exchange, you'll paste your API key in. Entering your API key will populate the MailChimp List drop down menu. If you have multiple lists, they will allow show in the drop down menu.
Then, if you'd like, you can change your Sign up Label text and check whether or not you'd like your customer to double opt-in for your email.
In the Advanced settings of your products, you can select a list to add your customers to based on the product they are purchasing.