Yay another mac user. . . I get same error too .. . btw my mac is a PPC running 10.5.8 just in case that makes the difference
here's the console message:
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] java.lang.UnsupportedClassVersionError: Bad version number in .class file
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.lang.ClassLoader.defineClass1(Native Method)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.lang.ClassLoader.defineClass(ClassLoader.java:676)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.security.AccessController.doPrivileged(Native Method)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.lang.ClassLoader.loadClass(ClassLoader.java:317)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:280)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
24/01/11 3:33:09 PM [0x0-0x2e02e].com.apple.JarLauncher[346] at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:375)
hope this helps in some way