Shell shell = styledText.getShell();

    // Create the menu
    Menu menu = new Menu(shell, SWT.POP_UP);

    // Create Cut
    MenuItem item = new MenuItem(menu, SWT.NULL);
    item.setText("Select All");
    item.setAccelerator(SWT.MOD1 + 'A');
    item.addSelectionListener(new SelectionAdapter() {
        public void widgetSelected(SelectionEvent event) {
            styledText.selectAll();
        }
    });

package com.wuntee.oter.view;

import org.eclipse.swt.SWT;
import org.eclipse.swt.custom.StyledText;
import org.eclipse.swt.events.SelectionAdapter;
import org.eclipse.swt.events.SelectionEvent;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Menu;
import org.eclipse.swt.widgets.MenuItem;
import org.eclipse.swt.widgets.Shell;

public class Test {

	protected Shell shell;

	/**
	 * Launch the application.
	 * @param args
	 */
	public static void main(String[] args) {
		try {
			Test window = new Test();
			window.open();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	/**
	 * Open the window.
	 */
	public void open() {
		Display display = Display.getDefault();
		createContents();
		shell.open();
		shell.layout();
		while (!shell.isDisposed()) {
			if (!display.readAndDispatch()) {
				display.sleep();
			}
		}
	}

	/**
	 * Create contents of the window.
	 */
	protected void createContents() {
		shell = new Shell();
		shell.setSize(450, 300);
		shell.setText("SWT Application");
		shell.setLayout(new GridLayout(1, false));
		
		Composite composite = new Composite(shell, SWT.NONE);
		composite.setLayout(new GridLayout(1, false));
		composite.setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true, 1, 1));
		
		StyledText styledText = new StyledText(composite, SWT.BORDER);
		styledText.setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true, 1, 1));

		addTextMenu(styledText);
	}
	
	private void addTextMenu(final StyledText styledText) {
		Shell shell = styledText.getShell();
		Menu menu = new Menu(shell, SWT.POP_UP);
		MenuItem item = new MenuItem(menu, SWT.NULL);
		item.setText("Select All");
		item.setAccelerator(SWT.MOD1 + 'A');
		item.addSelectionListener(new SelectionAdapter() {
			public void widgetSelected(SelectionEvent event) {
				styledText.selectAll();
			}
		});
		styledText.setMenu(menu);
	}
}