The path to the folder is a relative one, you can use a site relative path with a slash in it though if you need to. The default file field is for a default file, that is a file that will be used in place of the uploaded file if there is none or it fails.
The debug info is telling you that the file upload did not occur. What have you set your trigger to be and how are you testing the page? Please post back with this info and the page in question so we can take a look.