Попробуйте код ниже, обновив значение аргумента метода findElement:
//Find the main menu element
WebElement menu = driver.findElement(By.id("top_sell"));
//Perform mouse over action
Actions mouseMenu = new Actions(driver);
mouseMenu.moveToElement(menu).build().perform();
//Then find the sub menu element
WebElement subMenu = driver.findElement(By.cssSelector("a[href='/some_link'"));
//Now click on sub menu
subMenu.click();