The Workout Blender is functioning under Mountain Lion but still has a couple issues that we are currently trying to address. You can successfully make premixes, but once in a while the aspect ratio will change or the chapter markers might get out of sync. These are both things we are actively working on and are at the top of the bug list. If you don't mind the video being squished or stretched once in a while, you can use it until the next version is released.