As of May 12th, 2019, MailChimp's app that integrates with Shopify is no longer available. Here's Shopify's reasoning. They recommend you uninstall the app from your store by that date, and use one of their recommended alternatives.
For those that don't want to part with MailChimp, Shopify suggests trying one of these email integration apps.
Shopify is also working on a tool that will allow you to easily import your MailChimp lists into Shopify. Until that's ready, you can follow these steps to do it manually.
Our newsletter forms and popup add customer emails to your Shopify customers list and are tagged with Accepts marketing. Our support team isn't able to help with direct integrations with your newsletter services, so if you need some hands on help we suggest checking out TaskHusky,Blend Commerce, or HeyCarson for something like that. They’re specialize in smaller theme customizations and integrations like this.