The class is in the package applet.simpleapp. You try to call it as SimpleApp, but because it is in the package applet.simpleapp the classes real name is applet.simpleapp.SimpleApp (not just SimpleApp, the 'real name' is called the fully specified name and is how you should refer to it from the HTML). In addition, the class SimpleApp must be in a directory structure that reflects the package it is in. So you should create a subfolder called applet, and subfolder of that called simpleapp, and put the SimpleApp.class file there:
I get it now. So I should put the code="applet.simpleapp.SimpleApp" in the html code instead of just SimpleApp. And I also need to put the html code on the top directory along with applet directory, in which contains simpleapp directory, in which contains SimpleApp.class.