Thank you for this extremely quick and thorough advice. I have downloaded Onyx for Snowleopard (correct spelling this time). It has instructed me simply to drag the icon to my Applications folder which I have done. I cannot open it by double clicking etc. to see what I have, so must presume it just gets on with the job - unless I have missed something here!
Thank you, that was what I was doing wrong - 'wrong' icon chosen. I also had downloaded the snowleopard version which is the original on this Mac, but had not taken into account that I had recently updated to 10.9.5. This has now been accepted.