It means that if you are using a PC and not a MAC (Machintosh or however it is spelled), you have to go to the folder "C:\Program Files\World of Warcraft\Interface\Addons" and put the addon folder there.
Open the archive file (addon) if it is packed and check that the first folder in the archive is the addon folder and not the folder "Interface". If it is the folder "Interface" unoack it to "C:\Program Files\World of Warcraft\"