what kind of mods are you both using? if the mod is meant change the appearance of something, normally a XNB file, it replaces a file in a specific location. If your downloading a mod like timespeed it will have a manifest. the readme or the page you downloaded it from will tell you where to put it. if your files are still zipped - you will need a program like winrar/7-zip in order to unpack it and use it. Editted due to a poor explanation XD