The Shopify Lite/Starter plan does not give you access to your HTML files.
Using this Shopify plan? Follow this guide to install the Kudobuzz widget on your product pages.
1. From your Kudobuzz dashboard, click on Customize Widgets >>> Review Form Widget.
Copy the piece of code on the page.
2. Head over to your Shopify admin. Click on Products and select any of your products.
Click on the symbol <> and paste in the code.
3. Visit the page of the product. In Chrome or Firefox, place "view-source:" in front of the URL.
You will see a bunch of code showing up. Don't be overwhelmed, this is just your page in computer language.
Command/Ctrl+F and search for "rid".
Copy the number you find.
4. Head back to the product page in your admin and replace "product.id" in the code you originally posted with the number you copied.