If you do not use custom install directories Blizzard changed the default install directory after a recent patch. The game will ask you before doing this move and you can choose not to. I suggest allowing it to happen.
The old default directory was C:\Program Files\World of Warcraft
The new directory after updating WoW to current and accepting the directory change is C:\Users\Games\World of Warcraft
There is a chance the old directory may still be there which is usually what causes the confusion when installing addons.
There is no need to make any folders. Just make sure you install addons in the correct addons folder. To find the correct location you can Right-Click the shortcut and select "Open File Location" from the drop down menu. If that option is not available selecting properties from that drop down menu will give you the location to the correct install directory. It will be the directory listed in the "Start In" box.
If you need any help installing addons you can look at this
Thread/FAQ
Hope this helps.