If you see the following message, "The id field specified has not been mapped and is required", you will need to ensure that you have mapped the Constituent ID correctly (please note that you do have to create a merge field in Mailchimp in order to properly link these fields).
As well as mapping the Constituent ID in the mapping you will need to map it in the separate Constituent ID box at the bottom of the mapping screen when using an export (see below).
