Hi Madhumitha- Julianne from Mixpanel Support here!
The Mailchimp integration to send events to Mixpanel is built by Mailchimp. You can see their docs for setting up the events here. If you've followed those steps and still don't see the events flowing to Mixpanel, I'd recommend reaching out to the Mailchimp support team. From the Mixpanel end, we accept whatever Mailchimp generates and sends to us for these users, so we are a bit limited in the insight we have on what's happening.