First of all, when posting code here, you should put it between [code] and [/code] to preserve indentation and aid readability.
Also, your thread should be in the Programming forum, so I've reported it for moving there.
Have you compiled the code with javac? If so, are you in the same directory in which Hello.class resides? If not, you should be able to give the argument -cp (or -classpath) and give the path to the directory containing Hello.class. I think you can also set the CLASSPATH environment variable.