yeah steam problems....
im certain there are solutions on the site somewhere here.
when you chose the path on your own - did it install it for you?
if so - go to the steam folder -> beyond the swords = > mods
see if the star trek is there , than see if its in another sub folder, if so, move all the inner files to the out side - meaning cancel the sub folder.
than load the mod from the mods load options in civ4 or add a short cut through your steam - check the options - look for run options - you can add something like mod=star trek, im not sure, i can check.
if that wont help - try to unzip the installer file with 7 zip manually. than, put the folder of star trek in the mods folder as i wrote above.
dont worry, we will get to the buttom of this.
i got steam version also.