This line will not work but I think I have used correct xpath?
driver.findElement(By.xpath("//a[contains(@class,'cke_button_bold')]")).click();
to locate a button like below :
<a id="cke_73" class="cke_off cke_button_bold">
id is a dynamic number so can be used as fixed locator here. And class is a compound class which is not supported by WebDriver findElement method…
I created simple html file and your xpath works with FirefoxDriver in WebDriver 2.1.0.
Also you can try to use
Classname is supported by webdriver Api