1. You should have properly installed Java Plug-in (see http://www.javasoft.com/products/plugin/).
  2. Start Java Plug-in Properties (On Windows: Start/Settings/Control Panel/Java Plug-in).
  3. Check Enable debug on Advanced tab and press Apply.
  4. Convert your ".html" files to the format used by Java Plug-in (see http://www.javasoft.com/products/plugin/1.3/docs/htmlconv.html).
  5. Open html page with applet in Netscape/Internet Explorer.
  6. Open "Attach to VM" dialog from NetBean?'s toolbar or menu Debug. Enter the value from Java Plug-in/Connection Address field ("4025") to the "Name" input line in Attach to VM dialog and press OK.
  7. NetBeans debugger should be connected to the VM used by Netscape/IE now.

Java Plug-in threads and thread groups can be accessed from Debugger Window/Thread Group's tab and debugger should be ready to stop on breakpoints.

For details on applets, see the NetBeans IDE 4.1 Tutorial for Applets.

Source: NetBeans User FAQ