You can do that by using a reference to the Menu instance in the checker
form.

Here is an example I have written. In this example I'm setting a reference
to the Menu ("this")  instance in the Checker instance. From now on I can
reach the Menu instance from the Checker instance and do the set_Opacity
call on the Menu instance from the Checker instance.

_____________________________________
Menu.jsl file:

import System.Drawing.*;
import System.Collections.*;
import System.ComponentModel.*;
import System.Windows.Forms.*;
import System.Data.*;

public class Menu extends System.Windows.Forms.Form
{
private Checker che = null;
private System.Windows.Forms.Button button1;
private System.ComponentModel.Container components = null;

public Menu()
{
 InitializeComponent();
}

protected void Dispose(boolean disposing)
{
 if (disposing)
 {
  if (components != null)
  {
   components.Dispose();
  }
 }
 super.Dispose(disposing);
}

private void InitializeComponent()
{
 this.button1 = new System.Windows.Forms.Button();
 this.SuspendLayout();
 this.button1.set_Location(new System.Drawing.Point(96, 104));
 this.button1.set_Name("button1");
 this.button1.set_TabIndex(0);
 this.button1.set_Text("Checker");
 this.button1.add_Click( new System.EventHandler(this.button1_Click) );
 this.set_AutoScaleBaseSize(new System.Drawing.Size(5, 13));
 this.set_ClientSize(new System.Drawing.Size(292, 266));
 this.get_Controls().Add(this.button1);
 this.set_Name("Form1");
 this.set_Text("Form1");
 this.ResumeLayout(false);
}

/** @attribute System.STAThread() */
public static void main(String[] args)
{
 Application.Run(new Menu());
}

private void button1_Click (Object sender, System.EventArgs e)
{
 this.che = new Checker();
 this.che.parentForm = this;
 this.che.Show();
 this.set_Opacity( .50 );
}
}

_____________________________________
Checker.jsl file:

import System.Drawing.*;
import System.Collections.*;
import System.ComponentModel.*;
import System.Windows.Forms.*;

public class Checker extends System.Windows.Forms.Form
{
private System.Windows.Forms.Button button1;
public Menu parentForm = null;

private System.ComponentModel.Container components = null;

public Checker()
{
 InitializeComponent();
}

protected void Dispose(boolean disposing)
{
 if (disposing)
 {
  if (components != null)
  {
   components.Dispose();
  }
 }
 super.Dispose(disposing);
}

private void InitializeComponent()
{
 this.button1 = new System.Windows.Forms.Button();
 this.SuspendLayout();
 this.button1.set_Location(new System.Drawing.Point(72, 56));
 this.button1.set_Name("button1");
 this.button1.set_TabIndex(0);
 this.button1.set_Text("Close");
 this.button1.add_Click( new System.EventHandler(this.button1_Click) );
 this.set_AutoScaleBaseSize(new System.Drawing.Size(5, 13));
 this.set_ClientSize(new System.Drawing.Size(292, 266));
 this.get_Controls().Add(this.button1);
 this.set_Name("Checker");
 this.set_Text("Checker");
 this.ResumeLayout(false);

}

private void button1_Click (Object sender, System.EventArgs e)
{
 this.parentForm.set_Opacity( 1 );
 this.Close();
}
}

Regards,
Lars-Inge T?nnessen
http://www.larsinge.com