First things first,
What version do you have, is it Silent Hunter IV or the one with U-Boat missions addon?
If you only have SH4 you should update it to patch 1.4 wich can be found here:
http://www.subsim.com/radioroom/down...p?do=file&id=8
However, you should try and find the U-boat addon which is version 1.5, most MOD's are for 1.5 version and will not work with 1.4.
You can find a copy of the U-boat addon by going to Amazon.com (Link is on the top right of this forum).
When you've desided what MOD's you want to use check the MOD makers release thread and the README's for additional information on compatibility README's can be found within the downloaded MOD.
Hope this helps you in your choices.
HJ