If you wish to integrate your Mailchimp account with your Viral Loops campaign, simply follow these steps:
Go to the 'Integrations' step in your campaign's wizard.
Find the Mailchimp integration and enable it.
Then click on connect
Then click on allow
When you are setting up the new Mailchimp integration, you can choose which Audience to use from your Mailchimp account.
To learn more about what the integration does, check out this article: Mailchimp Integration 101