You are gonna kick yourself when you realise what you
did wrong..

[SNIP]

Your code was difficult to work with, it wold not compile
easily for me, and I had to add a 'main()' and change
components and layouts just to see the problem.  In future,
please consider submitting only SSCCE's, as they are a lot
easier for others to debug.
<http://www.physci.org/codes/sscce.jsp>

Having said that, after a few tweaks to your code, I got
it to run and fixed the problem you saw.  Look carefully
at the comments, and note that *this* is an SSCCE.
<sscce>
import java.awt.*;
import java.awt.event.*;
import java.text.*;
import javax.swing.*;
import javax.swing.text.*;
import javax.swing.border.*;
import java.util.*;

public class DateTimeChooser extends JDialog
{
    Vector hours = new Vector();
    Vector minutes = new Vector();
    JButton ok;
    JTextComponent component;
    // cjange to a JPanel for compilation
    // DateChooserPanel dateChooser;
    JPanel dateChooser;
    JPanel border;
    JPanel okTimePanel;
    JPanel okPanel;

    public DateTimeChooser(JDialog owner, JTextComponent component,
String title)
    {
        super(owner);

        this.component = component;
        DecimalFormat twoPlaces = new DecimalFormat("00");
        for(int i = 0; i < 24; i++)
            hours.add(twoPlaces.format(i));
        for(int i = 0; i < 60; i++)
            minutes.add(twoPlaces.format(i));

        //ok.addActionListener(this);
        ok = new JButton("Ok");
        ok.addActionListener(new ActionListener() {
                                 public void actionPerformed(ActionEvent e)
                                 {
                                     jButton1_actionPerformed(e);
                                 }
                             }
        );

        border = new JPanel();

        border.setBorder( BorderFactory.createRaisedBevelBorder());
        border.setLayout(new BorderLayout());

        //super( owner, title, true );
        //dateChooser = new DateChooserPanel();
        // change to a generic panel with borderlayout..
        dateChooser = new JPanel();
        dateChooser.setBorder(new EmptyBorder(5,5,5,5));

        this.setSize(160,120);
        this.setLocation((int)(component.getLocation().getX() +
owner.getX()),

(int)(component.getLocation().getY()+(component.getHeight()) +
owner.getY()));

        this.setUndecorated(true);

        border.add(dateChooser, BorderLayout.WEST);

        okTimePanel = new JPanel();
        okTimePanel.setLayout( new BorderLayout());

        JPanel timePanel = new JPanel();
        timePanel.setLayout( new FlowLayout());

        timePanel.add(new JComboBox(hours));
        timePanel.add(new JComboBox(minutes));

        timePanel.setBorder( new EmptyBorder(5,5,5,5));

        okTimePanel.add(timePanel, BorderLayout.NORTH);

        okPanel = new JPanel(new BorderLayout());
        //okPanel.setLayout(new CenterLayout());

        // you just went to all the trouble ot configure the
        // 'ok' butoon, now you are not going to *use* it???
        //okPanel.add(new JButton(" Ok "));
        okPanel.add(ok);
        okPanel.setBorder(new EmptyBorder(5,5,5,5));
        okTimePanel.add(okPanel, BorderLayout.SOUTH);

        border.add( okTimePanel, BorderLayout.CENTER);

        this.setLayout( new BorderLayout());
        this.getContentPane().add(border,BorderLayout.CENTER);
        this.setFocusable(true);
        this.pack();
        this.setVisible(true);
        this.toFront();
    }

    private void jButton1_actionPerformed(ActionEvent e)
    {
        System.out.println("Finally works");
    }

    public static void main(String[] args) {
        JFrame f  = new JFrame();
        f.pack();
        f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        JDialog d1 = new JDialog(f);
        d1.setVisible(true);
        DateTimeChooser dtc = new DateTimeChooser(d1,
            new JTextArea(), "Problem?");
    }

}
</sscce>