Once you've added the unit (there's a readme included in the file, and tons of tutorials floating around here as well to help you with that) Open up your editor, open up you're scenario, open up Rules, Edit rules, and go to General Settings. On the right side it should say Barbarian Sea Unit, and if you haven't changed it alread it'll say under that Galley. Just click that tab and select the Ghost Ship, or whatever you've named it in your scenario.