Erro na execução

Está ocorrendo um erro ao rodar o exemplo da primeira aula.

Erro : The geckodriver.exe file does not exist in the current directory or in a directory on the PATH environment variable. The driver can be downloaded at https://github.com/mozilla/geckodriver/releases.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using OpenQA.Selenium;
using OpenQA.Selenium.Firefox;

namespace TesteAutomatizado
{
    class Program
    {
        static void Main(string[] args)
        {
            IWebDriver firefox = new FirefoxDriver();
            firefox.Navigate();

            firefox.Navigate().GoToUrl("http://www.google.com");

            IWebElement caixaDeTexto = firefox.FindElement(By.Name("q"));

            caixaDeTexto.SendKeys("Caelum");

        }
    }
}