What I did was to download the JB-Pro zip file again. I managed to butcher it, before I discovered that by default you move a file, instead of copy it, on a Mac.
Next, I ran jAlbum, with the Lite version of Juicebox installed. Then, I followed your instructions in the readme.txt that is included in the skin download (visible after changing that to a .zip file):
"To open the skin's folder, go to 'Tools -> Open Directories -> Skin directory' from jAlbum's drop-down menu."
That gave me a Finder window in the proper location. That location, by the way, is:
However, I can't seem to navigate to it myself, so I'm doing something wrong with Finder, or maybe it's some sort of alias, or maybe it's hidden to me. Doesn't matter, I got there courtesy of that menu instruction.
[Update: yeah, that particular Library folder is normal invisible. I installed Invisible -- https://www.macupdate.com/app/mac/47448/invisible -- which is easier than running assorted terminal commands. Now I can navigate directly to that location myself, but it's still easier to let jAlbum open it for me, as opposed to trying to remember that location myself.]
Then I closed jAlbum. Didn't want the skin open, but Finder was left open.
Then I ran another Finder, and opened the downloaded JB-Pro zip file. Now, having researched drag-and-copy better, I just dragged the jbcore from that Finder window to the one that was showing the skins folder (having navigated to the /juicebox/jbcore area).
Oh, yeah, I replaced spinner.gif there, too.
This was actually pretty easy, i.e., let jAlbum open a folder for me, and use the jbcore from the zip file for JB-Pro. I never did find out where jbcore is residing permanently, as part of JuiceboxBuilder-Pro, and I looked.
So, good suggestions you gave me, but it was ultimately the simple part of readme.txt that steered me in the right direction.