If you like to insert Mailchimp Subscriber form under footer widget, you need to install the recommended plugin Mailchimp for WP.
Under Mailchimp for WP -> Form, paste in the following code to style your form to match the theme style.
Make Sure you have created an API Key from mailchimp.com. If you are beginner in mailchimp, here is beginner’s guide tutorial you can follow.
After you created the form on Mailchimp for WP you will find a shorcode, copy that and goto Appearance->Widgets and under “Footer Left” widget area add a text widget and paste the shortcode.