You’ll have to create a new API Key in MailChimp and copy/paste it into TntConnect, replacing your old/revoked one.

To do this, pull up "Sync with MailChimp" and click "Options". 

You'll see a link to "Where is my API Key".  Get a new API key and paste it into TntConnect.

You need to create a new API key.  Click "API Dashboard":

Then click the button to "Create a Key":

Then copy /paste that new API key into TntConnect over top of the old one.