first, try opening it again. it won't work, but do it anyways. close the error message.
next, click the apple symbol in the top bar, go to system settings/system preferences, and click on the 'privacy and security' category. Scroll to the bottom, it SHOULD say "ShadowDatingSimulator was downloaded from the internet blah blah blah" the important thing is to click on "Open Anyways." It should work, if it doesn't let me know!
Apple does this for any internet downloaded game, you unfortunately gotta do this process every time.