Cornerstone does not yet officially support Kaleidoscope 2, so to work with the latest beta of Kaleidoscope 2 (Build 99 and up) here is how to get set up:
Follow the instructions here Cornerstone External Compare Tools under the heading Custom Compare Scripts
The custom scripts contents should be
#!/usr/bin/env bash /usr/local/bin/ksdiff --partial-changeset -- "$1" "$2"
The script you create needs to be executable for Cornerstone to use it, so at your command line:
chmod +x /path/to/script
This works with latest Kaleidoscope 2 Mac App Store version as well.
Sent an email to Zennaware in regards to Kaleidoscope 2 support in Cornerstone.
They are aware of it and planning to integrate it into Cornerstone but there is no specific date yet.