You definitely clicked Fots 2.2 - MEGA download (Mac) from the options on the game page? The file hosted on Itch.io itself is PC only, so I doubt it could install on a mac.
For reasons I don't entirely fathom, Itch-io also sometimes won't let downloads happen if it's an external link. Try the direct MEGA download here: https://mega.nz/#!gvA0mSJA!Zu9-ophla4L1WCReybsvNClk8ljXXSieXVSGiU6tZEg and let me know if you have any luck with that.
If the download is working fine but the actual installation doesn't work I'm not sure I can help, as I don't own a mac, and FotS isn't really intended to run on that system.