This can be tricky if you try it by using by the normal id or class name. You can try doing this it worked fine for me:-
You need to specify more info to the a tag saying which value to select. So you need to use something like :
driver.findElement(By.xpath("//a[@title='Selenium']")).click();
In this way you actually are looking for the search box first and then you look for the suggestion drop down by clicking on the element and its done. You get your drop down bar. Hope it helps.