I found a similar error as mine on stackoverflow and added selenium webdriver jar files to the project using the below method :
right click on project--> goto build path--> configure build path--> click on "Add external jars"--> add selenium jar files from your local machine--> click ok--> now mouseover on WebDriver in your code--> click "import webdriver"--now run your code-->you will get rid of the exception.
However, I am still getting an error. Here's the error :
Exception in thread "main" java.lang.Error: Unresolved compilation problems: WebDriver cannot be resolved to a type FirefoxDriver cannot be resolved to a type
This error happens when you use Eclipse as IDE and try to run code that doesn't even compile. Check your Problems view in Eclipse, and fix the compilation errors before executing the application.