I believe there may have been some server maintenance carried out yesterday which may have affected uploading files.
If you try again today using dropbox or the song upload widget on the dashboard page and still have no luck making it work I would recommend opening a support ticket for a quicker response.
DropBox was in fact performing maintenance on their network, which affected our ability to pull the tracks in to our system. They didn't do a very good job at informing their partner developers, though, which made it harder to track down the issue on our end. All is well again.