The plugin is a standard J2Store application, installs as any other Joomla plugin and can be managed both from usual plugin area or from the Apps section in the J2Store setup.

If correctly configured, collects and passes to the selected Constant Contact list the first name, last name and email address of the buyer. It is triggered by the onJ2StoreAfterCreateNewOrder event.

The setup is simple, there are 3 parameters to be filled:

Constant Contact API Key Enter your Constant Contact Authorization API Key. 
Constant Contact Authorization Token Enter your Constant Contact Authorization Token. 
Membership list Constant Contact Membership list ID

You can obtain the API Key and the Authorization Token you need to register an account at https://constantcontact.mashery.com and link it to your main Constant Contact account.

You can find out your Membership List ID in the same place, in API Tester Tab go to "Retrieve a collection of ContactLists" then click on Try it, It will show you all the existing List IDs.