There aren't too many "submarine" modders, so it takes a bit of time for someone to see your posts. I do mostly "text" kind of stuff, and try to stay away from ships and planes and submarines for now, since I am really good at making the game CTD, sometimes even taking the computer down...
In the meantime, there is already a Sen Toku in the game, an AI Japanese submarine. If you're trying to copy that, you might well have a "conflict" between the two boats, to the point that the game doesn't know which boat you are referring to. Also, more importantly, what are your dates set to in the configuration of your version, and what date(s) are you trying to use it? If you load the Mission Editor, do you get an error message as it loads about the Sen Toku not being found in the library?... and if you don't, can you add the submarine to a mission file there?