This is a common issue on Oracle's Java 7 on Mac OS X 10.7 and 10.8.
The simplest solution is to go back to using Apple's Java version 6. You can do it like this:
To uninstall the Java 7 internet plugin (Java 7 JRE):
Go to /Library/Internet Plug-Ins/ and delete the JavaAppletPlugin.plugin bundle.
To uninstall the Java 7 JVM bundle (Java 7 JDK):
Go to /System/Library/Java/JavaVirtualMachines/ and delete anything whose name starts with jdk1.7.0
Install a Java 6 runtime:
Open
http://www.java.com/en/download/testjava.jspYour browser will show a security warning. Click the button to allow the Java applet to run.
You will be asked to install Java 6 for OS X from Apple.
Now you should be able to Play Planet Mule again
