Summary
To install ICC profiles placed them in this folder "~/Library/ColorSync/Profiles" and restart Capture One.
In-Depth
Uncompress the Files:
Typically, ICC profiles are available within zipped packages to compress file size. Unzip the package by double clicking prior to installing as neither the operating system nor Capture One can parse a zipped ICC profile.
Copy the ICC Profile file
Click the profile to highlight it, then select Edit > Copy to copy the file
Paste the fil to the macOS profiles directory
Open Finder, then select Go > Go To Folder
In the resulting dialog, type "~/Library/ColorSync/Profiles" without quotes
Select Edit > Paste Item to paste the previously copied file to that folder
If you are prompted for your user name and password, simply enter it and proceed.
Restart Capture One
As Capture One parses these folders for usable profiles on launch, it is necessary to relaunch Capture One such that the application "sees" the new profile.
The new profile can now be used as a Proof Profile, via View > Proof Profile
as well as a destination profile for the processed file, (which is set under the Process Recipe's Basic tab) or as a printer profile (in the Print dialog's Color Profile option).
If the profile does not appear on the first try, ensure that the file is present in the Profiles folder. If it is, reboot and relaunch Capture One.