Are you opening the .app file? If you try to launch the .exe it won't work on Mac, as the .exe file is the Windows build.
I've had a few people report that my Mac builds don't work on Itchio, but it always turned out to be that the person was trying to open the wrong file.
Viewing post in All Idleness and Ephemera comments
Yep, it was the .app file but for some reason it won't open. The symbol for the game pops up a split second when I try to launch the app, but it also disappears just as swiftly without anything else showing up on screen. I did however notice that I got a steam key with my purchase so I'm going to try launch it through that and see if I'll have any better luck there :)
The Steam version of Idleness is 100% identical to the Itchio version, so if the Itchio version doesn't work I'm doubtful that the Steam version would. I can try and take another look at it later, but as I don't have a Mac it will be more or less impossible for me to troubleshoot any issues on my own.