Setting the player's submarine

[LukeFF]

I am trying to set up a single-player mission with the Gato class sub, but the game lists every submarine but the Gato in the main single-player mission menu. I've made sure the box is checked allowing the player to use the Gato, and the date for the mission is after the Gato was introduced, but nonetheless I can't get the mission to work right! What am I doing wrong?