Oh strange! I just tested it out on my end and it seems to be working. Just to make sure we're on the same page, did you...
1. Select the white "Download Now" button
2. Either donate a random amount or bypass the donation section by clicking "No thanks, just take me to the downloads"
3. Select the mac option and start the download
4. Locate it in your downloads folder and unzip the zip file
5. Then click the renpy file named Crimson Hydrangea to start the game?
If you did all of that and are running into additional errors, you might need to either adjust your security settings OR troubleshoot with renpy since that's the program I used to create the game. I was able to download and have it play on my mac so I think it should work.
Viewing post in Crimson Hydrangea (Day 2 Demo) comments
Hmmm, do you have a downloads folder? I'd recommend searching for a file called CrimsonHydrangea-0.1-mac.zip . Once you've located that you can right click and click " open with" and select "archive utility" in order to unzip that file. Then once it's unzipped, you should see a new file appear labeled CrimsonHydrangea and once you double click the game should start playing.
If it still doesn't work I'd recommend either downloading or troubleshooting with https://www.renpy.org/ to make sure that it's compatible with your mac.