ah so what you have to do is try and delete all the files of the game uve previously downloaded. then redownload it. the reason it wont open as the mac default unzipper breaks it. therefore u need to download a 3rd party unarchiver (see attached) and use that. as soon as the unarchiver has downloaded, open it (drag to applications, accept that its a web downloaded app etc) then go to the games .zip file, right click (ctrl click), go to open with, click the unarchiver, and it shld open!
hope this helps!
dont be afraid to ask for anymore help :)
