Go to 'Settings -> Integrations'. Select Mailchimp from the list of integrations.
Click the "Connect to service' button. This will open Mailchimp. Enter your credentials to login to Mailchimp. If you have multiple Mailchimp accounts connected to your login then you must also select which account to connect to.
You will then return to MakePlans to select which MakePlans account to connect to. Select your account.
As a final step you will have to select which list in Mailchimp you want MakePlans to add new subscribers to. Click on a list and you will see "This service is connected." next to it.
That's it. MakePlans will now automatically add a new subscriber in your Mailchimp list whenever a new booking is made.