Lorraine, sorry, I cant see what make or model machine your query relates to.
But generally overlocker attachments for sewing machines are fairly average in effectiveness. An overlocker or serger is a purpose built machine with sharp blades like scissors that trim the fabric then two loopers and two needles (or more) to form the overlocked stitch on the fabric edge. Trying to replicate this with a sewing machine is never going to be that effective and sergers have come down in price significantly for domestic use, you should be able to pick up a second hand one for maybe $200 or so. But if you view some of the questions regarding sergers on FixYa, you will see that people do struggle with mastering them as they operate completely differently to a sewing machine. I always suggest taking a class to learn threading up, how to do a rolled hem, how to diagnose issues and learning how to use one.
Walking foot is generally attached to the pressure foot as normal but has a little arm that you place over the needle bar, this is moved up and down as the needle goes up and down and this activates the walking foot to help feed fabric from the top too, giving more even stitching of thick quilts etc. So you need to identify the type of foot your machine currently takes and then see if there is a walking foot suitable for it. If it is a "low shank" then this one may fit, http://www.sewingpartsonline.com/walking-foot-low-shank-p60444.aspx
Or start here http://www.sewingpartsonline.com/sewing-machine-presser-feet.aspx
and see if you can identify the type of foot your machine uses, then see if there is a suitable walking foot. However, it may be easier to take your machine to a local dealer so he can put the foot on and confirm it will actually work on your machine, as where the needle bar and foot are is critical.
Trust this helps you a little.