- Go to Manage --> Advanced Settings --> Site Settings --> services -> newsletters and punch in your API keys
- Push the update button at the bottom and Impact Stack will find all the lists you've created in the current email marketing tool
- Remove all old newsletter checkboxes from your forms
- drag in the "new" newsletter field on actions that just appeared (it's in the "generic pallette"), edit the field and under the "options" tag you should now see all your email lists
- Choose the list you want these people to subscribe to and click save
Note for Mailchimp: if you create new fields in Mailchimp with the same form key as the contact in Impact Stack, then it will auto-sync for all standard Impact Stack contact fields.