If your Contacts List screen has been customized, the MailChimp integration may not be visible.
You can turn it on for the your Contacts grid, but you will need to ensure the following columns are added to the grid:
The columns do not need to be visible, but they do need to exist in the grid.
Enabling the Send to MailChimp option:
1. Navigate to your Contacts List screen in the Contacts App.
2. Click the gear in the upper right corner and select Customize.
3. Within the ListSection section, select the ContactsGrid control.
4. On the right properties panel, under Settings, uncheck the checkbox Hide Send to MailChimp.
5. Save the screen.
MailChimp is now available from the export icon.