The Skyrms simulators run on the Java 1.1 event model. So if you get an error message that says something like

Applet skyrms2 can't start: error: java.lang.NoClassDefFoundError: skyrms2 java.awt.event.(etc.)

then you need to either upgrade your browser to explorer or netscape 4.0 or later, or you need to go to
http://java.sun.com/products/jdk/1.1/jre/download-jre-windows.html
at the sun website and get a newer set of java class files to install.

Updating your browser is probably easier, and overdue as well.