In the Options tab, you will find some additional settings where you can add files that will get displayed when the form is submitted.
Tick "Send notification email to the sender" if you want to send a copy of email to the sender. Also read How to send a copy of email to the sender? above.
Tick "Display download links" if you want to display download links after the form is submitted. This will also include download links in the notification email sent to the sender.
In the Mailchimp tab, enter your MailChimp API Key and List ID. Then press Refresh and map the form fields to your MailChimp merge tags.
Note: Make sure your Mailchimp tags match the form fields to avoid subscription failure. It's fairly straightforward if you use ordinary text fields. It can get a bit tricky once you start using options in your Mailchimp fields. If you've used options within your Mailchimp fields (such as radio buttons or dropdowns), make sure the values sent through FormPlus match the options in your Mailchimp. In order to set this up correctly, use the same field types and options in your FormPlus block.
In the Mailchimp tab, there are 3 opt-in options for you to choose from. Simply select the one that is relevant to you.