With the actions object you should first move the menu title, and then move to the popup menu item and click it. Don't forget to call actions.perform()
at the end. Here's some sample Java code:
Actions actions = new Actions(driver);
WebElement menuHoverLink = driver.findElement(By.linkText("Menu heading"));
actions.moveToElement(menuHoverLink);
WebElement subLink = driver.findElement(By.cssSelector("#headerMenu .subLink"));
actions.moveToElement(subLink);
actions.click();
actions.perform();