The path.txt file is right in the Space Empires IV Gold directory. Change the entry from "none" to "Star Trek Mod" without the quotes. This will start the game with the Star Trek Mod loaded.
Alternate option:
Here is a link to the SE4 mod launcher:
http://forum.shrapnelgames.com/newup...1039630481.zip
Unzip the program into your SE4 directory and create a desktop shortcut for it. Then open the SE4 launcher and you can pick any mod you have installed and it will load it for you without having to change the path.txt file!