At the moment, the On-Guard program is available only for Microsoft Windows platforms (Windows XP or later); it is (in theory, at least), possible to create a version for the Macintosh - if there is sufficient demand - and even for Linux.
If you are interested in such platforms, please feel free to post here.
More importantly, if you are a C++ programmer with experience in applications development for either the Macintosh or Linux platforms, we would be very happy to work with you in developing versions for these platforms.