My Elna SU will only do straight stitching it will not zig zag

I have the same model, have you turned the first knob on left, the stitch selector, to the stitch you want? Then turn the middle knob, the stitch width to "4". Use the colour guide marks shown in the little picture on left of the knobs so you are dialing up the right settings. So for the zig zag, elastic blind hem, scallop, overcasting and multi-stretch stiches, 1-6 on the diagram you'll be setting the left knob to 1, 2, 3, 4, 5 and 6 respectively and the stitch width and length knobs to "green", you can vary the length and width to suit what you want.

Then for the cams, you need to set everything zero, turn stitch selector to 7, the cam setting. then turn the width to orange again (3-4) and the stitch lenth selector to the orange setting, right around the knob. This engages the "Elnagraph" the device under the flap on top of the machine to read the discs. Basically the green stitches the needle is swinging from side to side while the feed dogs move the fabric forward, making your zig zags, blind hems etc.
But when you engage the Elna graph, the machine matches stiches by moving the fabric "forward and backwards at certain times, while the needle does its thing from side to side. This is what makes elastic or stretchy type stitches because you are putting "stretch" into the seam by sewing forwards and backwards.

Please note: always turn your stitch width and length back to Zero BEFORE you make any changes to the left knob and select a new stitch as this saves on wear and tear to the moving parts in these wonderful machines.

I do hope this assists you and I haven't just told you stuff you already knew!