If you are not getting your leads delivered to your MailChimp account and have already configured the deliveries within ContactUs.com admin panel, probably your MailChimp credentials could be incorrect.
The credentials you need are an API KEY and a List ID. We highly suggest you to verify if they're the right ones.
If you don't know how to retrieve them, here is a quick guide on how to find them on MailChimp:
API KEY: Access your MailChimp account > Click on your username > Account Settings > Extras > API Keys.
List ID: Access your MailChimp account > Lists > Click on the list name that you want to use > Settings > List Name & Defaults.
When you have your credentials, you will need to access your ContactUs.com admin panel.
From there you will go to Form Settings > Configure Form > Delivery Options > 3rd Party Integrations > MailChimp. Click on MailChimp's logo and you will need to copy/paste the credentials. Enable the integration and save.