How do you insert a bobbin in Singer 5160C sewing machine?

Answer 1

That is a very tricky proceedure. It is so hard to explain, without showing, you should probably take your machine to the repair shop and ask them to show you. They can also check out the machine and make sure all the parts are in good working order.

If you want to see how a sewing machine works, this link takes you to an animated show. I really learned something:
http://home.howstuffworks.com/sewing-machine2.htm