What OS are you running on the computer? What folder did you install Silent Hunter to?
Refer to
JScones' JSGME Thread and maybe
MikeMike's JSGME Thread and
The JSGME Download here threads for downloading and using JSGME (Jones Soft Generic Mod Enabler), which is definitely the way to go. Follow JScones' advice, and read the documentation for JSGME. Folder structure is *VERY* important (hence my questions about your install of SH4).
When you're looking for mods, you would use the ones for v1.5 of Silent Hunter, of course. Will you be doing mainly the US Fleet Boats, or the Uboats?