Hello firestar12,
Well, it seems you have allready entered your license key in your profile.
But to go on multiplayer, you also have to patch your game to at least version 1.3, but preferably patch it to the latest version, which is 1.4.2. You can get the patch(es) here (http://www.shipsim.com/downloads/patches.php).
If you're unsure about which one you need, you can check your current version by looking in the lower lefthand side of the main menu in the game.
So then you would pick the patch that takes you from thát version, to version 1.4.2
Then, when your game is patched up, multiplayer is available from the main menu in the game.
Regards,
Fred