Here is the stitch selection page from your manual
I would suggest that you use the stitch called "elastic overlock" using a stretch needle, you need a ball point tip on the needle to press between the elastane fibres in lycra. A stretch needle is blue, coated with a substance that makes it easier to stitch these types of man made stretch fibres, often size 75. If you can't get one of those, then use a ball point needle, size 70 or 80.
You need a stitch with forwards and backwards movement so that the stitching doesn't break under stress. Your machine also has a stretch straight stitch and this would be a forwards and backwards straight stitch so this will work too but can look a bit 'heavy' sometimes. But even triple zig zag narrowed down will work often.
If you are sewing leg edges with lycra, shop made garments will have clear swimsuit elastic sewn into the seam using a coverstitch overlocker, you'll never get as good a result with a regular sewing machine but you can get a reasonable finish with practice. This elastic is available to buy, you might want to use the overlock stitch to attach it to the fabric edge, stretching as you go, then turn it to the inside and use a stretch twin needle and straight stitch to top stitch from the outside, stretching the fabric as you sew.
Trust this helps you a little, 4 thumbs up if so. :-)