Please navigate to "apps.freshdesk.com" and search for MailChimp to install this. Kindly make sure you have a MailChimp account to integrate with Freshdesk.
Once this is successfully added to your helpdesk, the MailChimp widget would be available inside the contacts under the customers' tab.
For instance, you click on that widget inside a contact say, Alex - a window with two tabs namely, campaigns and mailing lists would open with several options underneath that would allow you to select which to subscribe and unsubscribe appropriately.