WordCrusher is PC only, but as a Mac user myself, what I did was run Windows on my Mac using Parallels, which costs about $80 but frequently goes on sale in various Mac bundles etc so you can usually get it for around $45, often bundled with other good apps. There are some other virtualization software packages that I haven’t any experience with that you could also use to run Windows on your Mac, and you can also use Bootcamp, which comes with your Mac and allows you to boot into Windows instead of into OS X. If you scroll down on the WordCrusher download page, the final video is a webinar that includes some advanced formatting and Mac alternatives, which also includes links to all of the above.
Alternatively, use the Book Format Rocket which will work on both PC or Mac. (Unless you are creating a fully illustrated children’s book in which case you should use the free Kindle Kids Book Creator.)