This is because your account is subscribed to a tangible-goods-only plan level which provides no upload storage space and no support for selling digital products such as downloads. To change your plan to a level that supports digital products, please see the instructions on this page:
...and be sure to Subscribe to a plan that provides disc space for product files.
When you Add Product and enable the Single File Download setting, after you click Submit to save the new product's settings, you will find the Upload Product File button on the final screen where we present button codes for that product.
If you have already added a product and want to add or replace the file for that product, then when you Edit Product, the Upload Product File (or Overwrite Product File) button will appear on the first screen of settings for that product.