The upload button won't appear if the Single File Download option was not checked earlier in your product settings. Please also note that certain other settings such as Redirection or Package Files will automatically disable Single File Download because they are incompatible.
If you've already added your product but were not able to upload a file at the time you can easily fix that by editing the product. Go to Seller Admin > View/Edit/Delete Products, select the product and press Edit.
As long as the Single File Download box is checked on the first page an upload button will appear on that very same page, allowing you to upload the file as intended. Be sure to save your changes afterward by pressing Next until you can Submit them.