Source Filmmaker is not available for Mac but there are a few alternatives that runs on macOS with similar functionality. The best Mac alternative is SFM installer for Mac OS X. It's not free, so if you're looking for a free alternative, you could try Storyboarder. If that doesn't suit you, our users have ranked five alternatives to Source Filmmaker and three of them are available for Mac so hopefully you can find a suitable replacement.
Storyboarder makes it easy to visualize a story as fast you can draw stick figures. Quickly draw to test if a story idea works. Create and show animatics to others. Express your story idea without making a movie.