As explained, many times already on the forum, when you don't have buildings at all, it's NOT a problem of the activation. If the scenery is not active for any reason, the scenery will not just disappear, it will run in TRIAL mode.
This means, your problem doesn't have anything to do with activation.
First, be sure you are using the *current* installer, because if you purchased years ago, if you are using the old one didn't work with Vista or Windows 7.
Try with the current installer first and, after installation, verify you see the "Addon Manager" menu on the main FS9 menu bar.