If you have installed the Pre-Order app on your Shopify store, activated the button in the product settings, you should be seeing the button replace your "add to cart" button.
But if you do not see the button, please follow these steps:
Go the app dashboard, click on Products
See if you have toggled the switch to green for the pre-order products
Ensure the pre-order status is shown as active
If you're still not seeing the pre-order button, please reach out to us for support
The pre-order app supports all Shopify themes. But if for some reason you believe it is not working with yours, please reach out to our support team for help in setting it up.