Java 6 isn’t installed by default. Java 6 is available as a simple Software Update, so if your system is up to date, Java 6 is installed, else upgrade your system. Note that Java SE 6 won’t appear for users on 32-bit Intel machines (Intel Core and Intel Core Duo) even if their systems are fully up to date.
Even though Java 6 is installed, Java 5 is still used by Mac OS X. You have to activate Java 6 by yourself. To do this, you need to run :
Applications -> Utilities -> Java -> Java Preferences
You will get the following window :
You just need to change the order of Java versions to use (Java application versions). Once Java SE 6 is on the top of the list, it should be activated.
You can check that Java 6 is correctly activated. You need to run a terminal (Applications -> Utilities -> Terminal) and to type the command: java -version