Mailchimp List API not integrating

Hi, I have been trying to get the Mailchimp integration to work so that members upon registering to our site may also opt-in for our monthly Newsletter. However when I run a test with a new sign-up, the user does not appear in my Mailchimp list of subscribers. I have added the Mailchimp API, and tested with adding a List ID to all subscriber levels and still no luck. I have not enabled the debugging as the site is live, however I can run a staging site, but would a new member produced in the staging site show up in Mailchimp? Also, I do not want to automatically add any member to mailchimp. Instead, it would be preferred (and moral) for them to sign themselves up, so is it also necessary to add a List ID to a Membership level? (by the way, level 0 is a unapproved member, level 1 is approved, all membership to the site is free). This is a multi-network website, if that is important. Anyone have any advice on how to get this working? Thanks!

Kind regards,
Jon