How do I troubleshoot an error with mailchimp plugin when adding a new user?

I get the following error when I add a new user in the WP admin section.
I installed the mailchimp plugin and added an api code in the settings, I must have more to set up...

Warning: MailChimp listSubscribe() Error: 104 - API Key can not be blank in /home/content/64/10578864/html/wp/mtg-stuff/plugins/mailchimp-sync/mailchimp-sync.php on line 120

Warning: Cannot modify header information - headers already sent by (output started at /home/content/64/10578864/html/wp/mtg-stuff/plugins/mailchimp-sync/mailchimp-sync.php:120) in /home/content/64/10578864/html/wp/wp-includes/pluggable.php on line 876