After you install the Wishlist PRO app to your store, you need to install the Wishlist PRO snippet into your shop's theme. Once this is done, the snippet code has to be added to the product page of your store. You can do so by going to:
Your shopify store admin dashboard→ Apps→ Wishlist PRO→ Installation→ Click on “Install Snippet” → Copy the code snippet → Open your templates/product.liquid (sometimes snippets/productform.liquid) file → place your snippet code to the product page of your store. (Usually under h1 tags or somewhere near the title).