Throughout the Swiss Elna range there were several of their top of line models that have the black cams for additional stitches and they all have an "Elnagraph" device in the top of the machine case where you insert the cams.
There are single cams where the needle just moves ffrom side to side while the fabric moves forward. And double cams where the needle moves from side to side while the feed dogs move the fabric forward and backwards to get a combination stitch. The trick is to get the Elnagraph to engage so the arm in it can "read" off the disc.
There are a couple of other knobs that you need to adjust to get the cam stitch though.
Star Series: (white machine with blue top) you need to put the stitch selector inside the elnagraph flap to "A" on the Star series and then stitch width to 4 and stitch length to the red A zone.
Air Electronic SU or Carina: you have 3 knobs from left to right - stitch selector (put it on 7) , stitch width (put it on 4, orange area) and stitch length (turn it around to the orange zone for the automatic stitches).
And on the early Supermatic the adjustments are a little different again.
And ALWAYS turn the stitch width and length dials back to zero, before you change the stitch selector, or pop out a cam.
I hope this helps you get your Elna stitching from the cam, there is a great Yahoo group called Elna Heirlooms too where there are lots of knowledgeable Elna owners and some files listing all the cam stitches.