Hello!
Jesse from the Guru Team here.
You can find the install instructions for Product Reviews here: http://shopify.com/admin/apps/product-reviews/install_instructions
I installed a fresh copy of React in my test store and worked through the install steps. It looks like you actually need to paste your code in the snippets/product-grid-item.liquid
That should take care of it.
Feel free to reach out at any time if you have any other questions!