Download and install the Java2 Runtime Environment (J2SE JRE) from java.sun.com if you do not already have it installed.
All you need to do is double click the jar file to start Ladder.
Some systems may not support executable jar files correctly. If double clicking on the jar file does not work, try the following command lines:
java -jar ladder.jar
java -classpath ladder.jar com.Ostermiller.Ladder.Ladder