Ever since Windows Vista came out, there have been troubles with the older DirectX 9 games running in Windows. The main thing is the install folder. If you chose a "default" Steam install, it put the game in "C:\Program Files (x86)\Steam \SteamApps \common \Silent Hunter Wolves of the Pacific". Windows will "protect" that folder from you and other intruders with UAC (User Access Control) and other 'modern' OS 'technologies'. You will need to move the game install. If you have a 2nd viable hard drive (not an archive drive), then you can create a 2nd Steam Library folder and install SH4 there. If however, you only have a C: drive, then you will have to move Steam and the game. Consult with
Moving a Steam Installation and Games on the Steam Knowledgebase web site. If you don't move the game, Windows will rollback any cfg or other perceived "system" files to their original state, leaving you with a messed-up game install, half mod, half not.
Once you get past that hurdle and get the mod activated, you do have to clear the Save folder, which defaults to "C:\Users \UserName \ Documents \SH4". You can navigate to the Documents folder and delete the SH4 folder, or navigate into the folder and delete the "data" folder inside of it. SH4 runs off of the information in that folder, and does re-create the folder each time it starts, if the folder is missing or empty. The game will not overwrite existing information in the folder though, so when it encounters an existing "NSS_Porpoise.sim" file (as an example), it leaves it alone. That means that the RFB file will not overwrite the Stock file, so you have to delete the old data.
When you get into the larger mods, such as FotRSU, then a 64-bit memory "manager", such as LAA or NTCore get thrown into the fray. They can also help with the stock game and modded game's "efficiency". All they do is let the OS know that the SH4.exe file can address more memory, hence the "Large Address Aware" LAA name. Mandatory for FotRSU, recommended on TMO, etc. For the CapnScurvy paradigm on all of the above, consult his
Windows10; Large Address Aware; SH4 Install thread.