Your API (application programming interface) key is the key which the addin uses to log on to mailchimp and get your data. You can find it in your mailchimp account on-line as follows:
1. Log in to your MailChimp account
2. Select the drop down with the name of your account on the top right hand side of the mailchimp window. Then select 'Account' .
3. In the next screen select 'Extras' and then API keys
3. You should see an API in the list. If not select 'Create a Key'
4. Copy the key, and paste into your MailChimp add-in .
5. Select 'Test Connection and if you have the correct API key your application will say 'Everything is Chimpy'!