I have Left4Dead on the PC. The Only Mods that are working is Custom Maps. If it's Weapon Mods, other things like that. It does not work. Ironically Left4Dead 2 works fine with Mods. Left4Dead and Left4Dead 2 both have Add-on File where I put the Source Game Add content. Left4Dead 2 works fine with Steam Workshop and Mods but Left4Dead with Mods Yes or Not.