selenium2 and webdriver Alert

driver.findElement(By.xpath("//input[@value='添加']")).click(); //Pops out an Alert and program stops, does not continue 

how to click the alert?

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As of the latest selenium 2 release, this can be done (at least using the FirefoxDriver):
    driver.switchTo().alert().accept();

In previous version of Selenium 2 I have had no choice that to handle alerts in Internet Explorer by overriding the window.alert in Javascript:
IJavaScriptExecutor js = (IJavaScriptExecutor)driver;// Override window.alert to store the prompt and accept it automaticallyjs.ExecuteScript("window.alert = function(msg) { document.bAlert = true; document.lastAlert=msg; }");// Do some stuff...// Check for alertObject o = js.ExecuteScript("return document.bAlert");if (o != null && (bool)o == true){    //retrieve the alert message    o = js.ExecuteScript("return document.lastAlert");    // Do something with the alert text}

Selenium 2.0b3 has support for handling Alerts in IE and Firefox, so you can do the following:
IAlert alert = driver.SwitchTo().Alert();// Get the text from the alertstring alertText = alert.Text;// Accept the alertalert.Accept();
However, I have not been able to get the above to work with Yes/No alerts (Dismiss() works for No but Accept() doesn't work for Yes). I'm in the process of looking at the IEDriver to work out why this is.

In Selenium 2, currently alerts are only handled in the Firefox browser. You don't specify what language you're using for your tests, but here's how to handle an alert using ruby. (This is taken from the Ruby Bindings page on the selenium wiki).
Javascript Alert/Confirm
You can use webdriver to handle javascript alert and confirm dialogs. The implementation for both is the same.
Note: At this time the API is only available in Firefox (or in Firefox using the remote server), and only alert/confirms that are generated post load can be captured.
require "selenium-webdriver"driver = Selenium::WebDriver.for :firefoxdriver.navigate.to "http://mysite.com/page_with_alert.html"driver.find_element(:name, 'element_with_alert_javascript').clicka = driver.switch_to.alertif a.text == 'A value you are looking for'  a.dismisselse  a.acceptend

C# code:
IAlert alert = driver.SwitchTo().Alert();alert.Accept(); System.Threading.Thread.Sleep(milliseconds);

IWebDriver driver = new ChromeDriver();driver.Navigate().GoToUrl("http://skynet:8081/1.htm");var selenium = new WebDriverBackedSelenium(driver, driver.Url);selenium.Start();selenium.Click("css=input[type=button]");Assert.AreEqual(selenium.GetConfirmation(), "Are you sure?");Assert.AreEqual("OK", selenium.GetAlert());// <input type="button"  value="Alert test" />driver.Quit();

Alert alert = driver.switchTo().alert();alert.accept();
If you want to cancel the pop up use the following instead of alert.accept(): alert.dismiss();

You will have to handle exception and run your handler code for Alert, for Java:
      try{         driver.findElement(By.xpath("//input[@value='添加']")).click();       } catch(org.openqa.selenium.UnhandledAlertException e){         Alert alert = driver.switchTo().alert();         alert.accept();         // you logic here what you want to do next      }  
Catch this exception, and then you can accordingly accept or reject alert.