Go to Admin → Plugins → Email Service Providers, click Attentive, and select the green Activate button.
Add client_id, client_secret, and signUpSourceId from Attentive
Leave access_token blank. It will be obtained in the next step.
Enter an optional FriendlyName
Press Connect
Edit the plugin. Press the link at the bottom to obtain access_token.
Where to find your name and key in Attentive.