Darwin isn't available for users on a Wordpress.com free or personal plan since you cannot add Custom HTML or install plugins on these plans.
Login to Darwin's Dashboard and visit "Tracking Code" in the navigation.
Copy the tracking code HTML that you'll find there.
Login to WordPress dashboard and go to "Appearance > Editor" in the navigation.
Find your "Theme Header" (header.php) file and open it.
Add the tracking code in this file before
</head> closing tag and save changes.
Things that you should be aware of:
Once you've saved your changes, you can verify your install on the Darwin dashboard.