ERRO no Primeiro Exemplo

IWebDriver driver = new FirefoxDriver(); driver.Navigate().GoToUrl("http://www.google.com.br"); driver.FindElement(By.Name("q"));

        IWebElement campoTexto = driver.FindElement(By.Name("q"));
        campoTexto.SendKeys("Estacio");

        campoTexto.Submit();

O código acima ao ser executado apresenta erro na linha: IWebDriver driver = new FirefoxDriver();

Diz que: An unhandled exception of type 'OpenQA.Selenium.DriverServiceNotFoundException' occurred in WebDriver.dll

Additional information: 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.