I figured out a solution myself.
For anyone who may stumble upon this thread like I did, here's what I did to get Duke Plus to work. This is probably not the only solution but it was the one I used. Hopefully, these steps will get it working for you, too.
1. Download, install, and configure YANG to use EDuke32: http://yang-online.com/
2. Unpack the .zip or .rar to the directory where your EDuke32.app file is located! **Do NOT
unpack to Library/Application Support/etc.**
3. Launch YANG.
4. In the menu bar of YANG, select Action -> Single Player.
5. In the Extra arguments text box type -game_dir DukePlus
6. Click the Play button in YANG. Eduke32 should launch.
7. Check the Messages tab. One of the lines should read "Using DukePlus/ for game data"
8. Click the Start Button in EDuke32 to enjoy Duke Nukem 3D with the DukePlus mod!
A bit late but can confirm this edit to the .cfg file mentioned earlier works perfectly, tested on OS X 10.9 Mavericks:
if that isn't available to you try editing the eduke32.cfg file in the duke directory
change the line to read: ModDir = "DukePlus"
Change eduke32.cfg extension to .txt via 'Get info' if you can't open the file, make the edit then revert to .cfg.
I must be doing something wrong.
I have extracted everything that I downloaded in the DUKEPLUS.zip into \Library\Application Support\EDuke32. The DukePlus folder is now contained inside the folder at this location. I have attached a screenshot of what the inside of this folder looks like.
Then, I open eduke32.cfg with TextEdit and follow the directions quoted above and re-save the file.
When I run the EDuke32 app I don't see any particular DukePlus options and there are no DukePlus menu items once the game is running.
After I quit the game, I re-open eduke32.cfg and the file has reverted to:
ModDir = "/"
Can anyone point out my mistake? Thanks!