Ensure that Pre-order Now says pre-order is enabled for the product (screenshot below.) If it's not, follow the guidance in the app to enable pre-order.

If it says pre-order enabled but the button still isn't appearing, it's likely that Pre-order Now is unable to find the add to cart button on your product page. At this point, either contact support for assistance or, for those with HTML / CSS experience, continue on with the instructions below.

In the Chrome browser, go to your product page, right click on your add to cart button and click 'Inspect Element'.

Copy the ID (or class) of your add to cart button (screenshot below.) Remember that IDs need to start with # (e.g., #add-to-cart-button) and classes need to start with a period (e.g., .add-to-cart-button.)

In Pre-order Now, go to the Default Settings tab and paste the ID or class name in to the button selector field (screenshot below.)

Save changes.

Visit your product page and see if the pre-order button is appearing now. If it's not, please contact support for assistance.