When you run the app and the receipt is not present, it is supposed to prompt you with a login dialog so you can enter your sandbox account's email and password, and then it will generate a receipt and put it in the _MASReceipt folder for you. Except right now it looks like receipt generation is broken so the _MASReceipt folder never gets populated with a receipt, and you get an error saying your application is damaged and can't be opened.