In the SilentHunterIII directory ( C:\Program Files\Ubisoft\SilentHunterIII ) you will find JSGME already installed
Click on the program icon and you will see a list of available mods on the left hand side
Click a mod you want
Click enable in the middle column
And done
If you dont like it you click the mod in the right column and click disable
Simple
As you have already installed GWX dont forget to add the Dec 23rd update
http://files.filefront.com/GWX___Dec.../fileinfo.html
Download and extract into the mods folder in C:\Program Files\Ubisoft\SilentHunterIII and enable through JSGME
THIS SHOULD BE THE FIRST MOD YOU ENABLE
Make sure you are back in port before adding
Any problems just ask