• Download the mod from the host site.
• Extract the files to a folder on your desktop.
• Open your WoW folder.

If using a PC, the typical default install path for XP is:    C:\Program Files\World of Warcraft  or in Windows Vista and 7:   C:\Users\Public\Games\World of Warcraft

If using a Macintosh, the typical default install path is:  Macintosh Main HD (or whatever you renamed it to, if you did)/Applications/World of Warcraft

• Open that folder, create another one inside it, called AddOns (again, if it doesn’t already exist)
• Open the AddOns folder
• Copy the files from the folder on your desktop into the AddOns 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/ < ModCFolderNaem >/< files >

(where < ModAFolder > < .B. > < .C. > are the names of the actual folders that the mods create)

• Log into game. At the character select screen, there should be an "AddOns" button in the lower left corner. Click on that. Make sure the addon you added is checked ON either for all characters or just specific ones (drop down box at top left of window.)  Choose the "Load Outdated AddOns" checkbox at top right.
• Log the rest of the way into game, you should be up and running now, usually with a new minimap button.