1- Exit World of Warcraft completely
2- Download the addon you want to install
3- Extract the file – commonly known as ‘unzipping’ – Do this ONE FILE AT A TIME!
4- Verify your WoW Installation Path
5- Move to the Addon folder
6- Start World of Warcraft
7- Make sure AddOns are installed
The should be saved in a single level folder. One of the most common reason that UI's wont load is that there is a sub-folder inside the Custom UI Folder
Your final path should look like:
PC:
C:\Program Files\World of Warcraft\Interface\AddOns\< ModAFolderName >\< files >
C:\Program Files\World of Warcraft\Interface\AddOns\< ModBFolderName >\< files >
C:\Program Files\World of Warcraft\Interface\AddOns\< ModCFolderName >\< files >
Mac:
Macintosh HD/Applications/World of Warcraft/Interface/Addons/< ModAFolderName >/< files >
Macintosh HD/Applications/World of Warcraft/Interface/Addons/< ModBFolderName >/< files >
Macintosh HD/Applications/World of Warcraft/Interface/Addons/ < ModCFolderName >/< files >
(where < ModAFolder > < .B. > < .C. > are the names of the actual folders that the mods create)