Hi Luke welcome to Ship Simulator!!
In order to get yourself online you must first enter your serial number to your online profile. To do this, you have to go to your account settings here on the forum, and there should be a box for you to enter in your serial number. This will connect your online account with your copy of the game.
In order to download the add-on ships you must either download the patches found
here (http://www.shipsim.co.uk/downloads/patches.php). Alternatively, if you bought the ShipSim 06 add-on there is a free add-on you can download to convert that into ShipSim 08.
I hope this answers your questions, if you have anything else please let us know
Regards,
Adam