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
You just need to import two links:
import org.openqa.selenium.WebDriver; import org.openqa.selenium.firefox.FirefoxDriver;
or you can follow that tutorial:
https://www.youtube.com/watch?v=7yYDOja8n_k
It works for me.