Purchased the game, but after unzipping the file the exe just gives me a message saying "The application "DD_Mac" can't be opened. Is there any fix for this?

Hey there. 

Try one of these: - youtube.com/watch?v=sWSySXfR17c

Or it may be the unzipper. For Mac version try using this to unzip the game; https://theunarchiver.com/