If you have already added in your product information then you will want to go to Seller Admin>View/Edit/Delete Products, select one of the products and click Edit. On the page that come up, at the bottom, will be a button to Upload Product File. Once the file is uploaded, you will want to click Next until you can Submit your changes.
If you have not added in the product information yet, then you will want to go to Seller Admin>Add Product. Enter in the product information and then click Submit. The page that then comes up will include an Upload Product File button towards the top of the page.