To sync your contacts with Mailchimp you will need Admin permissions and you need to go to "MANAGE > ADMIN > SETTINGS > CORPORATE > IDENTITY" and then click on the branch you wish to sync by clicking on the pencil icon under Actions.
You will then want to enter in your Mailchimp API Key as well as how many days back you want to sync. If you leave blank it will sync all contacts for the entire branch.
To get your Mailchimp API Key login to Mailchimp and then go to the top right and click on your username and then "ACCOUNT" and from there click on "EXTRAS > API KEYS" then from there click "CREATE A KEY" button. The API key is a very long random code similar to this ff989ed586sw6586ecad0d63b59f588e3-us3