See the Mail Chimp Settings forms in the Admin under Configuration.
Note, using Mail Chimp "Merge Vars", you can have any callable methods on User sent to the MC list, mapped to indentically named (but uppercased) fields in the MC list admin console. Unfortunately these are only sent at the time the subscription is created, so there isn't much available.
Inspired by from https://github.com/softa/spree-mail-chimp thanks to Sam Beam sbeam@onsetcorps.net
Copyright (c) 2011 Print Bear, released under the New BSD License