Mac – I can’t seem to open this .jar file. Can someone make sense out of the error message

The problem actually was that Mountain Lion gives this nonsensical error message to any app not authorized by Apple or from the App Store, by default. You just have to go to Apple -> System Preferences... -> Security & Privacy -> Allow applications downloaded from:

Then select "Anywhere". Seems like this is useful, but I couldn't get Minecraft to run without disabling this security check.

In general, you run a ‘runnable JAR’ by opening it or with Terminal java -jar /path/to/game.jar.

However, this JAR does not contain a suitable manifest for launching, i.e. not a runnable JAR.

$ java -jar /path/to/game.jar  
no main manifest attribute, in /path/to/game.jar

Therefore you have to specify the main class manually. However, that still doesn't work.

$ java -cp /path/to/game.jar E
Exception in thread "main" java.lang.NoClassDefFoundError: javax/microedition/lcdui/Graphics
[…]
Caused by: java.lang.ClassNotFoundException: javax.microedition.lcdui.Graphics
[…]

That's because this JAR doesn't support being run on a computer. It's a game for a mobile device and you can't launch it on a computer.