The Steam version is OK for mods, but does cause some extra work for you. Do you have an older version from GamersGate, and then added v1.5 Uboat Missions download from Ubisoft on top of the disk install? I did that with one of my SH4 copies, and got that same message when trying to apply the Uboat Missions. But even if you've got the Gold Edition, contact Ubisoft Tech Support (support.ubisoft.com in US). You'll have to open a support case, which there is a button toward the bottom of the page to "Sign in...", from which you can create a log-in ID. I think you have to log-in again for follow-ups. A couple of years ago, it took them like two days to get back with me, and then asked me the same questions I had already supplied the answers to in the first support mail... but they did finally send me a new Uboat Missions download (which has since expired again...
), but at least they did respond, and they did get me going.