codebase="/classes/"

Mean it is using library at the same web site(where the web page is located!).
If you want to use library at other web site  you need to add something like
codebase="http://home.phy.ntnu.edu.tw/~lookang/classes/"

Each applet should have it's own name , if you add another applet tag with the same name in your previous post, the browser did not know which one to act on. That is the reason why it was not working.

You need to write special code to show applet tag instead of insert another applet into the message.

type [b]& lt;[/b] (without space between & and lt;)to show <