OS X Java Applet Log
With Windows it's quite easy to view a Java applet's console, especially if you only have one applet running. With OS X, it's a bit more difficult. You can, in the Java Preferences decide to show the log, but it will always pop up when you launch an applet.
Fortunately, so long as logging is enabled, you can view the log in OS X's Console application. Console allows you to view all the log files on your system in one place. Finding the Java applet log is a bit difficult, as it doesn't show up in the Java folders. Unfortunately in OS X 10.5.3, the applet messages are put into the generic system.log.