Thank you Dave, but I'm stumped already 
You said:
"This sounds like it might be a lack of permissions for the Plugins folder. Make sure you have FULL read and write permissions for that folder and then try installing the plugin again.
If the Library folder is hidden, you'll need to make it visible. Type ~Library in the search window of Finder, hold Option and hit Return.
Right click on the Plugins folder, choose Get Info. Unlock to allow changes and change for your log in to Read and Write"
Well, I went into Finder and did a search for '~Library' and nothing whatsoever came up. So I typed 'Library' and masses of options came up. I sorted them alphabetically and could only find 2 'Library' folders. One was empty, and the other nothing to do with the programs we're talking about.
I'm assuming the 'hidden' Library folder would ordinarily be visible in Finder? Anyway I'm not sure either, what the instruction "hold Option and hit Return" is executed? (I did say I'm not very computer literate, so what seems easy to someone like your good self, is never straightforward to someone like me - so I'd rather you talk in step-by-step language [not assuming anything, for I don't mind if you talk down to me, effectively]).
For the record, when I eventually (and hopefully) do get to see the folder you're talking about, do you mean select+hold AND Return at the same time, or something else? I find the whole process confusing.
Sorry to be a pain - and thanks in advance 
Ian