你需要添加一个 ActionListener 到 JButton 而不是一个 MouseListener . 请参阅如何使用按钮。
当 + 按钮，您想添加一个新的 JTextField 到 JPanel 包含允许用户输入玩家姓名的字段。
下面是根据您的问题中的代码改编的代码，它添加了 JTextField 当用户单击 + 按钮。关于代码的注解出现在代码后面。

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.WindowConstants;

public class PlayGame implements ActionListener, Runnable {
    private static final int  MAX_PLAYERS = 6;
    private static final String  BACK = "Back";
    private static final String  PLUS = "+";
    private static final String  START = "Start";

    private ArrayList<JTextField>  playerNameTextFields;
    private GridBagConstraints  gbc;
    private JFrame  frame;
    private JPanel  localButtonsPanel;

    public PlayGame() {
        playerNameTextFields = new ArrayList<>();
    }

    @Override // java.lang.Runnable
    public void run() {
        showGui();
    }

    @Override // java.awt.event.ActionListener
    public void actionPerformed(ActionEvent event) {
        String actionCommand = event.getActionCommand();
        switch (actionCommand) {
            case BACK:
                // Add code to execute when user clicks 'Back' button.
                break;
            case PLUS:
                addPlayerNameTextField();
                localButtonsPanel.revalidate();
                localButtonsPanel.repaint();
                if (playerNameTextFields.size() >= MAX_PLAYERS) {
                    ((JButton) event.getSource()).setEnabled(false);
                }
                break;
            case START:
                // Add code to execute when user clicks 'Start' button.
                break;
            default:
                System.out.println("Not implemented: " + actionCommand);
        }
    }

    private void addPlayerNameTextField() {
        JTextField playerNameTextField = new JTextField(10);
        playerNameTextField.setName("player" + playerNameTextFields.size());
        playerNameTextFields.add(playerNameTextField);
        gbc.gridy++;
        localButtonsPanel.add(playerNameTextField, gbc);
    }

    private JPanel createLocalPanel() {
        JPanel localPanel = new JPanel(new GridBagLayout());
        localPanel.setOpaque(false);
        GridBagConstraints c = new GridBagConstraints();
        c.gridx = 0;
        c.gridy = 0;
        c.insets.top = 10;
        localButtonsPanel = new JPanel(new GridBagLayout());
        gbc = new GridBagConstraints();
        gbc.gridx = 0;
        gbc.gridy = -1;
        gbc.insets.top = 10;
        localButtonsPanel.setPreferredSize(new Dimension(120, 200));
        localButtonsPanel.setOpaque(false);
        addPlayerNameTextField();
        addPlayerNameTextField();
        localPanel.add(localButtonsPanel, c);
        c.gridy = 1;
        JButton extraBtn = new JButton(PLUS);
        extraBtn.setFont(new Font("Segoe Script", Font.BOLD,40));
        extraBtn.setForeground(Color.WHITE);
        extraBtn.setContentAreaFilled(false);
        extraBtn.setBorderPainted(false);
        extraBtn.setFocusPainted(false);
        extraBtn.addActionListener(this);
        extraBtn.setToolTipText("Add another player");
        localPanel.add(extraBtn, c);
        c.gridy = 2;
        JButton startBtn = new JButton(START);
        startBtn.setForeground(Color.WHITE);
        startBtn.setFont(new Font("Segoe Script", Font.BOLD,40));
        startBtn.setContentAreaFilled(false);
        startBtn.setBorderPainted(false);
        startBtn.setFocusPainted(false);
        startBtn.addActionListener(this);
        localPanel.add(startBtn, c);
        JButton localBackBtn = new JButton(BACK);
        localBackBtn.setForeground(Color.WHITE);
        localBackBtn.setFont(new Font("Segoe Script", Font.BOLD,40));
        localBackBtn.setContentAreaFilled(false);
        localBackBtn.setBorderPainted(false);
        localBackBtn.setFocusPainted(false);
        localBackBtn.addActionListener(this);
        c.gridy = 3;
        localPanel.add(localBackBtn, c);
        return localPanel;
    }

    private void showGui() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.getContentPane().setBackground(Color.DARK_GRAY);
        frame.add(createLocalPanel(), BorderLayout.CENTER);
        frame.setSize(800, 600);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    /**
     * Start here.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new PlayGame());
    }
}

注意当你打电话的时候 setContentAreaFilled(false) 在 JButton ，不需要也打电话 setOpaque(false) .
而不是 new java.awt.Color(255, 255, 255) 你可以用常数 WHITE 在课堂上 java.awt.Color . GridLayout 以及 GridBagLayout 都不一样。参见在容器内布置部件
我没有像您那样使用背景图像，而是将背景颜色设置为深灰色。