I see that you raised the same query through a support ticket and I responded to it yesterday. I'm pasting the contents here for your reference:
The option to sync custom fields to MailChimp will appear only if you have at least one custom field created on your Chargebee site. Can you create the custom fields and then check if you're able to see that option? This feature doesn't require to be in any higher plan.
Apologies I raised the question at 2 places at same time. I was after quick response
However your last response was sufficient
thank you for your assistance