RFB is a mod, and is not necessary to play the stock game. If you want to use RFB, be certain you do not have the game installed in Program Files (x86) folder, but rather in a folder of your own creation, such a "C:\MyGames \Silent Hunter 4", or whatever. After that, the mods will activate properly.
Real Fleet Boat v2.0 (with patch) Download
Real Fleet Boat v2.0 Release Thread