First, check to see whether Java is installed by running this command in Terminal.app
If you don’t get a version number back, then you need to go here first and download the Java update from Apple: http://support.apple.com/kb/DL1515
Once Java is installed, then you need to enable it. In your Applications > Utilities folder, you should find the Java Preferences.app. Double click on that, and in the ‘General’ tab, click the enable checkbox at the top (see screenshot above).
Java working! 🙂