Image (click to enlarge):

Steps:

1. First, click on the blue folder icon for the category under which you wish to add a file.
    
2. Click the "Add File" icon.
   

3. Choose the file type you are looking for under "Files of Type".  You can choose to view text, image, sound, or all files.
    
4. Click beside "look in" to change the drive accessed.  Click the root folder to start at the beginning.
    
5. Double-click the folder where the file is located.  Continue to double-click sub-folders until you get to the right file.
    
6. Double-click on the icon for the file you wish to select.