To add a Facebook Pixel to your Mailchimp Landing Page, just follow the steps bellow:
Click on Brand, scroll down the page and then click on Manage Website;
Click on Update Site Tracking.
3. On the pop-up, turn on the Track with Facebook Pixel key.
4. Accept the Additional Terms and Conditions by checking the selection box. You just need to do it once.
5. Check the I have my own pixel selection box and digit your Facebook Pixel's ID.
6.Click on Save and you're done.