Having looked around for a focus stacking app for the MacBook I ended up installing the trial version of Zerene Stacker which gives 30 days full use before deciding whether to buy it.
My first attempt, eight focus stacked images of a flower on my Venus Flytrap:
Canon 50D, 70-200mm lens plus Raynox DCR-250 Super Macro Conversion Lens, LED ring light.
Not a startling image but I'm pleased with it as a first attempt working with jpg images.
I must admit I prefer the look of the program compared to CompineZP on Windows. You can see the final image being built up which gives a chance to see if any of the initial images might be causing problems. It was much easier to use and really flew through the align and stack process.