The reason why you didn't get a response to this in IRC is because 1. you spammed and 2. you were being a bit vague with your problem. The question here is still pretty vague.
Are you trying to drag unmapped mp3s into your osu! window to add them to your song list?
Are you getting some sort of error when you drag 'n' drop?
Are you trying to do something totally different, and I am misunderstanding you completely?
Describing your problem clearly will help us help you.