Hi there!! Unfortunately I'm not familiar with mac computers, but it should be the "forgetmenot.py" file to open up the game. You could also try the file in: "forgetmenot-2.0-all\forgetmenot.app\Contents\MacOS" and see if that one will open instead. Other than that...I'm unsure as to what the problem is. T_T You could try re-downloading the package as well and then retrying to see if it makes a difference too. Sorry if that's a little unhelpful! As a last resort and it's still not working, you can try playing the online version! http://sandbox-adventure.com/forget-me-not.html
Hopefully it works out!