To programmatically select/unselect HTML input type=checkbox element use the DOMInputElement.Checked property. The following sample demonstrates how to find checkbox element on the loaded web page and select it:

C#

using DotNetBrowser;
using DotNetBrowser.DOM;
using DotNetBrowser.Events;
using DotNetBrowser.WPF;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;

namespace DOMFormSample
{
    class Program
    {
        public class WindowMain : Window
        {
            private WPFBrowserView browserView;

            public WindowMain()
            {
                Browser browser = BrowserFactory.Create();
                browserView = new WPFBrowserView(browser);

                browser.FinishLoadingFrameEvent += delegate(object sender, FinishLoadingEventArgs e)
                {
                    if (e.IsMainFrame)
                    {
                        DOMDocument document = e.Browser.GetDocument();
                        DOMInputElement firstName = (DOMInputElement)document.GetElementByName("firstName");
                        DOMInputElement lastName = (DOMInputElement)document.GetElementByName("lastName");
                        DOMInputElement agreement = (DOMInputElement)document.GetElementByName("agreement");
                        firstName.Value = "John";
                        lastName.Value = "Doe";
                        agreement.Checked = true;
                    }
                };

                Content = browserView;

                Width = 1024;
                Height = 768;
                this.Loaded += WindowMain_Loaded;
            }

            void WindowMain_Loaded(object sender, RoutedEventArgs e)
            {
                browserView.Browser.LoadHTML("<html><body><form name=\"myForm\">" +
                "First name: <input type=\"text\" name=\"firstName\"/><br/>" +
                "Last name: <input type=\"text\" name=\"lastName\"/><br/>" +
                "<input type='checkbox' name='agreement' value='agreed'>I agree<br>" +
                "<input type=\"button\" value=\"Save\"/>" +
                "</form></body></html>");
            }

            [STAThread]
            public static void Main()
            {
                Application app = new Application();

                WindowMain wnd = new WindowMain();
                app.Run(wnd);

                var browser = wnd.browserView.Browser;
                wnd.browserView.Dispose();
                browser.Dispose();
            }
        }
    }
}

VB.NET

Imports System.Windows
Imports DotNetBrowser
Imports DotNetBrowser.DOM
Imports DotNetBrowser.WPF

Module Module1
    Public Class WindowMain
        Inherits Window

        Public Dim browserView As WPFBrowserView

        Public Sub New()

            Dim browser As Browser = BrowserFactory.Create()
            browserView = New WPFBrowserView(browser)

            AddHandler browser.FinishLoadingFrameEvent, sub(o, e)

                If e.IsMainFrame Then
                    Dim document As DOMDocument = e.Browser.GetDocument()
                    Dim firstName As DOMInputElement = CType(document.GetElementByName("firstName"), DOMInputElement)
                    Dim lastName As DOMInputElement = CType(document.GetElementByName("lastName"), DOMInputElement)
                    Dim agreement As DOMInputElement = CType(document.GetElementByName("agreement"), DOMInputElement)
                    firstName.Value = "John"
                    lastName.Value = "Doe"
                    agreement.Checked = True
                End If
            End sub

            Content = browserView

            Width = 1024
            Height = 768
            AddHandler Me.Loaded, AddressOf WindowMain_Loaded
        End Sub

        Private Sub WindowMain_Loaded(sender As Object, e As RoutedEventArgs)
            browserView.Browser.LoadHTML("<html><body><form name=""myForm"">" +
                                         "First name: <input type=""text"" name=""firstName""/><br/>" +
                                         "Last name: <input type=""text"" name=""lastName""/><br/>" +
                                         "<input type='checkbox' name='agreement' value='agreed'>I agree<br>" +
                                         "<input type=""button"" value=""Save""/>" +
                                         "</form></body></html>")
        End Sub
    End Class

    Sub Main()
        Dim app As Application = New Application()

        Dim wnd As WindowMain = New WindowMain()
        app.Run(wnd)

        Dim browser = wnd.browserView.Browser
        wnd.browserView.Dispose()
        browser.Dispose()
    End Sub
End Module