Downloadable products require that the files first be loaded into this interface.  The following tutorial will teach you how to upload a downloadable file for your products in OpenCart 1.5

Managing Downloadable Product files in OpenCart 1.5

1. Login to the Administration Dashboard as an Administrator
2. Place your mouse over CATALOG in the menu bar.  Click on DOWNLOADS when the drop-down menu appears.  Select a manufacturer from your list to EDIT. If you have no downloadable files already added, then use INSERT to add a new file.  To DELETE, select the item in the list, the select DELETE to remove it.
3. When you go to the downloads will see the following screen (if there are files, then they would be listed in a chart below):
   
   
   
   If you select INSERT, you will get the following screen:
   
   
   
   
   
4. Fill in the DOWNLOAD name.  If you have more than one language listing, then you will need both fields filled in because the DOWNLOAD NAME is required before it can be saved.
5. Next is the actual file.  Click on BROWSE in order to select the file that you wish to use as the DOWNLOAD.
6. The last item in the list is TOTAL DOWNLOADS ALLOWED.  This limits the number of times a customer can obtain the file via download per order.  If they purchase multiple orders and the number of downloads is greater than 1, then the number downloads cumulative based on the number of orders and number of total downloads allowed.
7. Select SAVE in order to save your entries
   
   
   

When the process is completed, the downloadable file is shown in the catalog for the product as per below:

For further information see How to Enable or Disable File Downloads in OpenCart 1.5.