Steam goes with Steam only. You want to use
Silent Hunter 4 Wolves of the Pacific: Uboat add on - Download (embedded US shopping link, benefits SubSim). I was going to link you to the US Ubisoft store, but they no longer sell it. It's probably available elsewhere though. The Gold Edition will "go on sale" at Amazon, usually coinciding with a Steam special...
Almost forgot my manners: Welcome to SubSim!