How do I merge fields between Mail Chimp Newsletter and Drupal User (Added First name. Last name)

Hi,

I have set up a Free form list in the Mail Chimp module for user roles ananymous, authenticated and admins and chosen a Mail Chimp list to connect to.

The Mail Chimp list has fields for First name and Last name, besides email.

I have added two extra fields to the Drupal User profile, First name and Last name. They show up when registering new users.

On the Edit a list page, under Merge fields, there are First name and Last name with dropdowns but from the choices there are none of the added fields of User.

How can I make the First name and Last name fields from User merge and appear in that dropdown?