E2M can be totally turned off and is by default. Don't Forget to turn it on.
You will require a Mailchimp API Key to allow E2M access to work with your account. (check Finding or generating your API key)
That key should be kept secret so E2M will crypt it before saving it to your site's database
Mailchimp users often decide to create different lists to segment their subscribers. For instance a list for buyers of product A and another list for buyers of product B.
That's a big mistake.
Every subscriber that buys both product A and B will be on both list and Mailchimp will count that unique user twice bringing you close to a (higher) paid account.
Instead it is best practice to create interest groups within one list and assign subscribers to those groups.
You can then add your customers to any of your interest groups
You will only be able to select a list once your API Key has been entered and updated (see Mailchimp API Key above.)
Custom fields (or merge vars) you have set up and attached to your list in Mailchimp can be filled automatically.
Once you have selected a list you have access to all your list's custom fields to choose from and which value to fill it with.
You can use values from user attributes, billing or shipping details, and even custom values.
Simply select the custom field to fill and apir it with a data source (user attributes, shipping or billing details...) and the data itself; and push the green Add button next to it.
You can add as many as you want.