1) A fresh installation of WoW does not always have an "Interface" folder, and rarely has an "Addons" folder. This usually has to be created by the user
2) Once WoW detects an "Addons" folder, it creates empty folders inside it named Blizzard_<stuff>.
3) In Windows Vista and Windows 7, having your WoW folder in the "Program Files" or "Program Files (x86) folder is not a good idea - it causes complications in where the addons/WTF/Cache folders are actually stored by the system.
4) When installing addons, often there is a problem called "folder in folder". The easiest way to detect this is to find a file for the addon named <addonname>.toc. For example, the addon Omen installs into a folder called "Omen", and inside the Omen folder is a file named "Omen.toc". That is, the path for the Omen.toc file looks like this:
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World of Warcraft\Interface\Addons\Omen\Omen.toc
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Folder-in-folder looks more like this:
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World of Warcraft\Interface\Addons\Omen-v3.1.0\Omen\Omen.toc
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The .toc file must be located in a direct subfolder of the "Addons" folder, basically. Interface\Addons\addonname\addonname.toc - anything else causes a problem.
If you have folder-in-folder, just move the 2nd folder (in the above example, "Omen") into the Addons folder, and this will fix the problem.