When using the using Manual Offline Activation to install Space Shuttle, the correct website to go to is: https://www.captainsim.com/activ/offline.php.
That's captainsim.COM, not captainsim.NET that you seem to be entering.

So if you use the link I provided, and you should be fine. ;)

Note. I just reinstalled my copy of the Space Shuttle in FSX, using the Manual Offline Activation, and it worked like it should for me. So it should work for you too.