I think you had this problem before:
http://80.95.161.114/shipsim/forum/index.php/topic,18186.msg245089.html#msg245089 (http://80.95.161.114/shipsim/forum/index.php/topic,18186.msg245089.html#msg245089)
If you solved it then, you should have written the key down.
If you were able to register and run SS 2008, you should see your serial key in your profile and I don’t think you should have to enter it again.