Knossos is available for download from the Knossos website.
Follow the instructions for your operating system to download and install Knossos. If an installer is not directly available for your operating system then it may be possible to build Knossos from the source code yourself. Follow the Github link on the Knossos website to learn more.
NOTE: Although Knossos is available for macOS, recent FreeSpace 2 Open versions are not due to a loss of the macOS build system. Future development of FreeSpace 2 Open for macOS may be limited, or outright impossible, however it should still be possible to create your own macOS builds for now.