After downloading and moving Kaleidoscope into the Application folder, you are ready to configure it to work with Sourcetree. First you need to navigate to the Integration section in the Welcome Window.
The ksdiff tool can now be setup by clicking the "Install Now" button.
In Sourcetree, choose Sourcetree > Settings…
Navigate to the Diff section in the toolbar. Select Kaleidoscope as Visual Diff Tool and Merge Tool.
And that's it, you're all set to start using Kaleidoscope together with Sourcetree.