I don't know if you've figured it out by now, but I just downloaded the game and finally got it to open after messing around with the Terminal a bit.
The "command line" can be found in Applications --> Utilities --> Terminal. Once the Terminal is opened you can type the command Quan Zillan left in the "read me" text ( xattr -dr com.apple.quarantine ). After typing that, DON'T add "Chitinous\ Carnival.app" Instead, drag the game file from your folder into the command line after "quarantine." My Terminal wasn't able to find the file when I typed the last part, but it DID find it after dragging the game file into it. The game should, hopefully, open after entering the command. Mine did at least.