Placing JPanel From A Class Into Frame Of Another Class

I can't figure out how to place a JPanel from a class(Game) in to a another class(Core). I have bean stuck on this for about three days now I bean doing my research but no luck so now I think I should ask directly so here I am. Now to state clearly I am trying to place a JPanel named Game_Screen witch is in another class named Game into a JFrame named frame witch is in a another class named Core.

Now both of this classes are with in the same package and my IDE Is Eclipse.The JPanel Game_Screen has other swing pieces attached to it that should also be added with the panel on to the frame.But the change is screen should only happen when I click the buttons New Legend(N),Load Legend(L),Help(H) what already happens is that when you click any of this buttons menu.setVisible(false); is activated making everything on screen at the time disappear from the screen but the panel does not a pear.

Here's my code:

//Class - Core-------
package Source;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Core {
static Game GG = new Game();
public static void main(String[] args) {
JFrame frame = new JFrame();

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