Just spend a couple of hours trying to get Pings to work with Mailchimp. I have read every post on using Pings with Mailchimp, as well as your video tutorial. It seems everyone is having problems.
Here are my settings:
Ping method = POST
I did choose to Ping to Subscribe to my list under the Membership Subscription Plan
I am getting a 200-OK message in the Ping History.
Mailchimp's API says the API call was received, method= lists/merge-vars, and Response = n/a.
Mailchimp referred me to WPMUDEV to solve this.