Does it have a loco/rolling stock on it? If not use Surveyor to build a consist (or more if the route requires), save the session (that's the route with the rolling stock) then open in Driver.
I suspect Nige is correct. If the route hasn't at least one piece of rolling stock on it, it won't show up in driver. It should show up in surveyor so that you can add some stock to it and then it should show up in Driver.