Yet, no matter where I put them, the mod would not show up.
Another note, I am not exactly youthful and my eyes are are not quite as good as they used to be, so I missed this until I really got to looking...
I finally saw it

When I use TextEdit.app to add the data="..." line, I finally noticed the fact that, when I typed the closing double-quote mark, just like the "helpful" capabilities in word processors, the text editor "helpfully" changed them to smart quotes. I then had to go in and "highlight" each smart quote and change them back to " (by just highlight them and typing over them - and NOT hitting a space bar - by clicking AWAY rather than space bar, I do not allow the TextEdit.app program to modify what I typed).
Anyway, when I did that, it worked. Then I decided to "double-check" it by doing the next one (ebartifact, Helm of Tohan). I opened up the configuration file and manually typed the line and, lo and behold, when I typed that closing double-quote, it changed them to be smart quotes on that new line - like this:
encoding=win1252
data="/Users/charlesmeade/Library/Application Support/openmw/data"
data="/Users/charlesmeade/Documents/openmw/masterindex"
data=“/Users/charlesmeade/Documents/openmw/ebartifact”
content=Morrowind.esm
content=Tribunal.esm
content=Bloodmoon.esm
content=master_index.esp
LOOK VERY CAREFULLY at the quote marks around the ebartifact data line - you will see they look SLIGHTLY DIFFERENT than the ones on the other lines.
Of course, this is NOT a problem with OpenMW - it is just a "helpful feature" (I mean that sarcastically) that thinks that it is OK for a text editor to behave like a word processor. Oh well, at least I figured it out - select EACH of those and make sure they are normal double-quote marks by overtyping them and making sure the normal double-quote mark appears.
Now, I will try it by copying the earlier line and only editing that last part (I am designating one place to put all mods that I want working like this).
Just thought it might help some future MAC user - to understand that the TextEdit.app is "helping" you in a very unhelpful way in this case.