I haven't had much luck getting other mailchimp extensions to work for me so I'm quite pleased to have this one up and running.
The only tricky part is knowing where to find your Mailchimp List ID. You can find this ID by logging into Mailchimp and choosing to view your list. In the URL at the top you will see your ID - which is the number at the end.
A big thanks to the developer for creating this and sharing it with the community.