Just upload your song. When its done click 'forum' under where it says Share this MP3. Then copy the link under where it says 'Share the download link for this mp3'... then paste it in the wiget. Remove the url things in front and behind the link when you put it in.
Ripway.com is a good alternative since I have tested it out. For this site, simply make an account, then click on my files, then upload a file. Upload an mp3 and then wait a while. It may say that you have no space, but ignore that. You will then get a direct link when it is done. You copy that link and paste it into the area in the mp3 widget settings.
Filefreak.com is also a good source. I am currently waiting for an activation email and will post the steps as soon as I get a chance.