No worries, and thanks for the helping hand. I admit I'm nearing the age when being bamboozled by the computer-savvy whizz-kids and their casual expertise becomes a fact of life.
What I did earlier, was copy the CM zip file to a newly-created 'mods' directory in the IAS home directory (I never did any 'installing' though, just extract the IAS files; not that I see any option to do so), because that's what I understood had to be done from the instructions in the OP.
This resulted in the mod properly loading, but with green placeholder sprites all over the place.
Trying the fix, neither of the data files worked (both giving the red crosses for missing images and sprites, as I understand), despite having copied in the diplomacy files. I also tried unzipping the mod file, and copying the diplomacy images directly into CM mod folders.
However, now I decided to read the conspicuous 'instructions.txt' file in the IAS folder, and extracted the mod files directly in there. After copying the data files it works 'fine'.
That is, all the sprites seem to be where they should, but the marines have a very ugly green background, and generally look pretty damn ugly, as if their colour palette was borked:
(that's 'unshaded' sprites; similar effect for the other data file)
I remember it looking much better back when.
By the way, some spellchecking would be nice here and there. Do you guys need a hand with that?