The plugin communicates with the MailChimp server to fetch your list(s) configuration and then caches this information for 24 hours. Should you have made any changes to your MailChimp lists which are not showing up in the plugin then you should manually tell the plugin to update your lists information.
1. Go to MailChimp for WP > MailChimp
2. Hit the “Renew MailChimp lists” button.
The plugin should then request your latest MailChimp list configuration and show a success message. Your latest lists information should show in the table below the button.
If you added new fields to your list you can now add them to the form via the buttons above the form editor under MailChimp for WP > Forms.