Hi there Nick,
It looks like you've already called in and Spoke to someone from our Success team who gave you an answer for this one, but just for the reference of anyone else who might stumble across this post.
At this time, the MailChimp integration is only available for Method Classic, and has not yet been rolled out for Method:New. While the MailChimp integration is certainly something we are considering, and is on our Product Roadmap, I don’t have an exact ETA on that one for you just yet.
In the Meantime - Have you had a chance to Take a Look at our Email Campaigns App?
While it doesn’t entirely replace all the functionality of a MailChimp integration, its certainly much more fleshed out than the Stock Campaigns offering was in Method:Classic.