Mailchimp will not sync the ContactUs.com form if you have any custom fields in your lists that are marked "Required". If you have the default MailChimp settings, then you won't have to change anything. However, if you have added any custom fields to your list, you need to uncheck any "required" fields you have on your account. Otherwise, they will not post to MailChimp from your ContactUs.com form. To complete this you need to login to your account (www.mailchimp.com), go to "Lists" on the top. Go to "Settings" => "List Fields and Merge Tags". Uncheck any custom fields you have created (i.e., anything besides Email, First Name and Last Name). Click "Save".