Let's be clear. Are you referring to Lightroom CC, or Lightroom Classic CC? What exact version number are you referring to? If you're talking about Lightroom CC (the cloud-based version) there isn't anything about external editors in the preferences. The only external editor available is Photoshop. As far as I know, there isn't a way to integrate the Nik plug-ins with the cloud version of Lightroom CC.
I am using LIGHTROOM CC Ver 1.4 on my MacBook Pro not the cloud version of CC
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Edit: I just reviewed your answer-
"I am using LIGHTROOM CC Ver 1.4 on my MacBook Pro not the cloud version of CC"
LIGHTROOM CC Ver 1.4 - IS THE CLOUD VERSION.!!
Lightroom CLASSIC-CC v7.4 is the DESKTOP version.
So still unsure what you have on your MacBook..
As Jim states- there are no "additional external editors" available in Lightroom-CC v1.4.
BUT- a work-around-
1. From the Right-click menu: Save the photo as a JPG (with Lightroom edits) to a folder on the hard-drive,
2. Open the folder with Finder
3. Select the saved photo
4. Open it with a NIK program. eg. Silver Efex Pro.
Most of the NIK programs can be used as "Stand-Alone" photo editors, so if you can find them in your Applications folder, you can make them available on the Mac dock and use with any photo saved to the hard-drive.
I just tested with NIK Silver Efex on my Windows-Laptop. Works OK.
Syncing the new 'NIK' edited photo to the cloud I am not sure about-