Go to Mailchimp Settings and enter your Mailchimp API key. Click on Save.
Then select "Export to Mailchimp" in Sync option.
Select the Subscribers list which you want to sync from Dryfta account.
Select the Mailchimp list to which you want to sync the subscribers list from Dryfta.
Click on Sync. That's it. Your subscribers would be exported to your Mailchimp account every 24 hours.

Video tutorial: https://www.youtube.com/watch?v=4OwDAeoxirA