Once you've uploaded the file you need you will locate the file at 'http://www.softlab.ece.ntua.gr/~sivann/pub/Uploaded/filename.filetype'
e.g. 'http://www.softlab.ece.ntua.gr/~sivann/pub/Uploaded/Paul_Bs_PES3_Compilation_Part_ 1.wmv'
Also notice how I've included underscores in the filename - its best to do it this way or stick to a one-worded filename - so that you don't have to type in "%20" which constitutes a space in the filename.