Open the zip file. Then use finder to locate "CH-DS" (it will probably be in your downloads folder or applications folder). Right click it, and press open. The first time it might tell you that apple cannot verify the dev, if this happens then just right click and click open again. This time it should have a menu in which you could click something along the lines of open anyway (it might try to fearmonger about security, that's just apple trying to vie for exclusivity).
After you've done this the game should open, and you can just open it normally every time in the future.