Are you trying to thread the bobbin for winding or for sewing?
For sewing, make sure the needle is fully up and then release the bobbin cover plate. Insert the bobbin so that the bobbin rotates counter-clockwise. Pull the thread through the slot at the center-front of the bobbin case. Hold down the bobbin with a finger then bring the thread to the left and through the slot on that side. You should here a clicking sound when you pull on the thread. Leave 6" of thread free and replace the cover plate. Refer to the manual (page 12) for pictures.
Many Singer sewing machine manuals are available for sale or download here: http://www.singerco.com/accessories/instruction-manuals
. The direct link to the manual for the 2662 is here: http://www.singerco.com/uploads/download/429_2662_efs.pdf
. See page 11 and 12.
If the bobbin thread isn't feeding, clean the hook and surrounding area. Then with the presser foot down, retry. If you aren't hearing the clicking sound when the thread is pulled, you may need the bobbin tension adjusted. There is a small set screw for the adjustment but it's really easy to damage the threads. If you still have a problem, the bobbin case or shuttle race may need to be replaced (See http://parts.singerco.com/CPpartCharts/2_2999/2662.pdf
for the parts breakdown).
I hope this helps.