SEIV on Steam: where should I place mods?

[Dogboy]

I bought my son a copy of SEIV for a game I'm setting up with friends. I bought it through steam, and we cannot find the se4.exe file. The se4 icon just activates steam. This is not true of my computer, where my steam-bought SEIV just installed a directory with the se4.exe file.

Has anyone had this problem? Where should I put mod folders? We're planning to play Atrocites' Star Trek Mod.

Thanks!