sourcemods folder missing and after creating it the mod doesn't appear in the game library

My friend and I were trying to play the portal cooperative mod for portal 1 and it involves installing the mod files to the sourcemods folder, I already had one but he didn't so he made one, spelt it correct all lower case, no space and he seemingly correctly installed the file to sourcemods but no matter what we do the Portal cooperative page doesn't appear in his library, any help would be appreciated