use this:
getAppletContext().showDocument(URL url,
String target);
The target argument is interpreted as follows:
"_self"
Show in the window and frame that contain the applet.
"_parent"
Show in the applet's parent frame. If the applet's frame has no parent frame, acts the same as "_self".
"_top"
Show in the top-level frame of the applet's window. If the applet's frame is the top-level frame, acts the same as "_self".
"_blank"
Show in a new, unnamed top-level window.
name
Show in the frame or window named name. If a target named name does not already exist, a new top-level window with the specified name is created, and the document is shown there.