There's a problem with Simmarket. We set the release date on Simmarket to be tomorrow, but somehow it already appeared on sale today, so the current GSX installer still doesn't know about it.
Your order and your Serial are valid though so, just wait when we'll release the new installer, which should be either later today or tomorrow.
And yes, we temporarily as soon we realized it went on sale too early. Simmarket has changed their developers website very recently, so I guess there are some quirks to be ironed out, such as the release date setting.