You need to connect your Mailchimp account to your PushPress account to sync the data.
Go to Apps in the main menu and select App Store
Find the Mailchimp tile and click Configure
Click Activate Mailchimp to turn it on
Find your Mailchimp API Key. Log onto Mailchimp.
In Mailchimp, go to 'Account' (bottom left by your Initial)
Select Extras -> API Keys
Create A Key
Copy this API Key (37 characters) from MC and paste into PP where it says 'Mailchimp API Key'
Select your Default Members List