How do I map lookups and master detail fields to Mailchimp merge fields?

In Salesforce, Lookup and Master-Detail fields create a relationship to a "parent" record. In the UX you see a clickable name, but in the underlying data, it is a field of type ID. IDs are 15 or 18-character alphanumeric strings. 

Data Wizard

The data wizard will map ID fields to Mailchimp text merge fields if you have included that field as a column in the source report or list.

• After running the data wizard,  the Mailchimp merge field will hold an alphanumeric ID  ("500P800000d2qgrIAA"), not the record name you see in Salesforce.
• To pass the text name, map a formula field instead.
• The data in Mailchimp will be up to date every time the wizard runs (up to hourly).

Field Mapping

Mailchimp to Salesforce: ID (lookup) fields can't be edited, so you can't map Mailchinp data over them.

Salesforce to Mailchimp:  This mapping is triggered by an edit on the contact or lead.

• You can't directly map ID fields to Mailchimp in the integration.
• Map formulas instead to send the ID or text name of that parent record (or other data on the parent).
• Note that edits on the parent record will not trigger the mapping sync. This is standard Salesforce functionality.
• To sync changes to a parent record or lookup, please use the Data Wizard, which runs on a scheduled not a trigger.

For further information on formulas or triggers, please see Salesforce documentation.