The button codes screen only shows an Upload Product File button when you first add a new product and are given that product's button codes for the first time.
If you have already added the product and returned to your Seller Admin, then you will find the Upload Product File button on the first screen of settings for that product. Just go to Seller Admin > View/Edit Product, select the relevant product from the menu and Edit, then you will see the Upload Product File button towards the lower-right, next to the product's Back/Next buttons.
If you had already uploaded a file for that product, this button would instead say Overwrite Product File (so you could replace a file you'd uploaded previously), and details about the last file uploaded for that product would be shown above the button in red text.