I think it's an ambiguous error message - one that i keep getting 'cus I dont do it right (either!)
Once you have "Browsed" for the file, you have to use the "Attach" button. It should then appear in the window above. Then fill in the details for submission, then hit the "Add Submission" button.
All should then be OK.
NB - Doesn't like .docx files at the minute - but maybe a set-up thing
Good luck!