There may be threads jammed up in the bobbin holder area causing the jam. But removing the inner bobbin holder is a bit tricky if you haven't done it before. There are two little screws holding the silver inner down onto a black ceramic piece (inner guard ring), and you need to get these lined up the right way to get the bobbin holder back into position afterwards.
I would suggest that you join the Yahoo group Elna Heirlooms, then check out the files and photos loaded up by other members who work on their Elnas.
You shouldn't need to remove the feed dogs, just give them a clean with a little brush. But internally the machine may need a good clean and lube and without a service manual or good guide this can be challenging.
If you go this link http://www.arizonaexpressionz.com/elna_service_manuals.html
you can down load the manual for this machine, the Star Series SU. Go to page 7 of the first part to see about removing the bobbin holder and inner guard ring.
It is a case of lining up the cut out of the black plate to the front, then putting the bobbin holder on top with the little arms facing front also, then replacing the two screws.
I'd suggest you take photos with a digital camera before you start to help you get it back in the right way.
I hope this helps you with the Supermatic, did you get the discs and accessories with it too? lovely Swiss quality machine with great stitching so if you can't get it unjammed, you may wish to find an experienced Elna Technician to take a look.
White Sewing Center stock lots of parts for these machines too, www.whitesewingcenter.com