Selenium WebDriver Java code doesn't execute on eclipse: Could not find the main class

I have installed both jdk6 (jdk1.6.0_26) and jdk7 (jdk1.7.0_25). I have two following java classes:

1st java file:

package code.google.com.p.selenium;
import org.openqa.selenium.WebElement;
public class GoogleSearchPage {
    // Here's the element
    private WebElement q;

    public void searchFor(String text) {
        // And here we use it. Note that it looks like we've
        // not properly instantiated it yet....
        q.sendKeys(text);
        q.submit();
    }
}

2nd java file:

package code.google.com.p.selenium;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.htmlunit.HtmlUnitDriver;
import org.openqa.selenium.support.PageFactory;

public class UsingGoogleSearchPage {
    public static void main(String[] args) {
        // Create a new instance of a driver
        WebDriver driver = new HtmlUnitDriver();

        // Navigate to the right place
        driver.get("http://www.google.com/");

        // Create a new instance of the search page class
        // and initialise any WebElement fields in it.
        GoogleSearchPage page = PageFactory.initElements(driver, GoogleSearchPage.class);

        // And now do the search.
        page.searchFor("Cheese");
    }
}

I am using Eclipse Indigo. In eclipse, I did the following steps:

1. Right click on UsingGoogleSearchPage.java
2. Click Run As->Java Application

The program was not executed. An error was occurred (Please see the image).