I use Selenium 2 WebDriver on Ruby.
How it is possible click on hidden link, with css (display: none)? the link is submenu and is visible when mouse over on menu.>
WebDriver emulates user actions, and doesn't allow clicking elements that a user wouldn't be able to click.
So you should do what a user would do: mouse over the menu before clicking. In Ruby you could do e.g.:
menu = driver.find_element(:id => "menu")
submenu = driver.find_element(:id => "submenu")
driver.action.move_to(menu).click(submenu).perform
The ActionBuilder class (returned by Driver#action) is documented here.