OK, you had it in the Program Files folder, which from Vista on through Windows 10 is a "protected" Windows System folder, which prevents you from making changes to files, such as adding a mod. The way mentioned in #7 used to work, but Steam now "protects" its files also, and does not recognize the game as a Steam game apparently, after the move. The way to do it is in a different post from that, but I don't remember which one... What you have to do is un-install the game as it is now. You might have to put the folder back where it was to accomplish that. Log into your Steam account, and work through there. You manage the uninstall and install through that, and you can put it anywhere you want. Just stay away from a Windows System folder, and you'll be OK.
Another point I forgot to ask you is whether you've got the "plain" version of Silent Hunter: Wolves of the Pacific, which is v1.3, or if you also got the U-Boat Missions add-on, which is v1.5? The UBM add-on is almost mandatory if you want to install mods. I cannot think of a single mod that's available for v1.3 anymore. There's only a few available for v1.4 anymore, and Steam doesn't allow an update to v1.4 for some strange reason anymore.
Edit: I found the correct post to use, which is #3 there:
http://www.subsim.com/radioroom/show...21&postcount=3