Please follow the steps below to add contacts to a Mailchimp List.
1. From the top toolbar menu, click Contacts > Contact List
2. Select the clients you wish to add to MailChimp by putting a checkmark beside their name (you can Select All in a certain category - MailChimp will not add duplicates)
3. Now select "Add to Mailchimp List" from the "Actions" drop-down box at the top of the screen.
4. Select the segment you would like to use or add a new one in the box below (segments are a way of categorizing the contacts for campaigns, and it is a required field).
5. Click on "Add to List" and wait until you see the "Finished" notification. That's it!