http://www.singerco.com/accessories/manuals.html
Here is a link for a free manual it is the second one listed just click the download button and you have it. It will have the threading instructions in there.
See I told you it was something simple, it takes an act of God to knock one out of time so if it isnt working right it is almost always a quick fix. It is a shame they can not build machines as sturdy now.
Billy