The app works based on a script tag and should be loaded and running after clicking Enable in the top section of the app page:

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To test the app, create a pre-order offer, and make sure you enabled preorder on your products, as noted here.

Once your offer is activated you should see the preorder button in your product page.

If you're not seeing it, you might need to add a custom selector to help the app identify your add to cart button.