Addon Folder should contain the Addon Files, minimum the .toc File that has the same name as the Folder!
Example Atlas:
C:\WoW_Nostalrius\Interface\AddOns\Atlas

Some Addons you download from Github/Bitbucket can have different folder names (like Atlas-Master) then the .toc file! you have to renamem them!
still doesnt work?
You can try to delete the Interface folder and reinstall the addons(start wow without addons once before - Addon Folder will be created).