Am I wrong? I thought the steam version cant use mods or the CS?
It can use mods fine. You need to redate the BSA files in order for texture replacers to work properly, and MGE won't work if the steam overlay is enabled, but other than that there aren't any problems with adding mods. The only other problem is that the Construction Set doesn't come with it so you can't make mods with it yourself unless you get the CS somewhere else.