Mac OS 10.6 comes with Java 6 pre-installed. However, if you upgraded an existing system to 10.6 it may not be the default.
For Mac OS 10.7 - no longer comes with Java installed!
You must make sure Java 6 is the default by moving Java 6 to the top of both the top and the bottom Java Preferences window. How you do that depends on your OS - here are some possibilities:
Finder > Applications > Utilities > Java > Java
Preferences
Macintosh HD > Applications > Utilities > Java Preferences.app
