Yeah, you need to get the version off of Steam. I had that problem when I started playing right after INS was released on Steam and was still using the manually installed version, so just download the one off of Steam and you should be fine.
Oh, and to get it, it should already be at the bottom of your My Games Tab along with about 5 other mods that they just went ahead and gave to everyone. If it's not then go to the Steam Store and find the Mods section.