To add the Darwin tracking code to your BigCommerce website, you need to use https. If you don't have SSL, the Darwin tracking code won't work right. Here is their guide on installing an SSL certificate on BigCommerce.
Login to Darwin's Dashboard and visit "Tracking Code" in the navigation.
Copy the tracking code HTML that you'll find there.
Login to BigCommerce and go to "Storefront".
Go to "Script Manager" page.
Click "Create a Script" button.
Once there, run through the following:
Once you've saved your changes, you can verify your install on the Darwin dashboard.