I'm a bit confused, this isn't a serger machine as far as I can see. So do you mean an overcasting foot where you sew a XXXXX type stitch along the edge of the fabric? The overcasting foot just helps you to line up the fabric edge and sew over it neatly.
if you go to this page http://www.sewingpartsonline.com/necchi-6022-sewing-machine-parts.aspx
there is an instruction manual for sale.
is also a great resouce on sewing machine feet with good photos of what they are used for.
Here is a video of overcasting foot in action http://www.sewingmachine-sales.co.uk/sewing_machine_info/video/34/adjustable-overcast-foot.html
If you are wanting to sew knit fabric then you just need to use a small zig zag stitch and a ball point needle to put elasticity into the seam. Or your model may have a stretch stitch, where the fabric is moved forwards and backwards during the stitch formation to add some elasticity.
Sewing elastic stretched onto fabric is a bit different with a SM, can be fiddly and hard to keep everything lined up nicely and stretched. Some machines have a little device which clips onto the needle plate in front of the needle that you can feed elastic through and tighten it to tension it, then just use a three step zig zag stitch to seam over this onto your fabric. But I don't know if Necchi has such a thing, your needle plate would need holes in front for it to clip into. Elna had these on their swiss machines in the 80's but haven't seen a similar device for a while.
Hope I've answered your question and not completely misunderstood your problem. Purchasing a manual is often the best way forward when a machine is unfamilar. Or www.sewing.about.com
is great resource.