At this stage, you've got some unzipped folders sitting in front of you, it's time to find them a home. You'll need to locate your World of Warcraft folder. I'll detail how this is done for a default installation, using the default file structure. Hopefully if you know enough to change the defaults, you won't need this guide. For Mac users, you will need to open a new Finder window, open up your Applications folder, then open your World of Warcraft folder. For PC users, you'll need to open the My Computer folder from your Start Menu. From there, open the hard disk that you have WoW installed on. (Default is C:) Next, open the Program Files folder, then locate your World of Warcraft folder.
Now that you have opened the World of Warcraft folder, you should see a folder named Interface. Open up this folder and you will find the Addons folder. This is the new home of your addons. This step is where some addons require some extra attention. Some addons, like Atlas and Cartographer, come with multiple modules that work together, but are located in different folders within the unzipped directory. Open up your unzipped addon folder and look for files with the .lua .toc file extension. (Example: Atlas.lua Atlas.toc) Once you locate a file with the .toc extension, you can take the folder containing that file and move it into the Addons folder. The .toc file is the Table of Contents file, which the World of Warcraft client needs in order to recognize and load each addon. (Thanks to Bloodieros for pointing this out and teaching me something I didn't know.)
As an example, I have downloaded the Atlas addon from Curse Gaming, after unzipping the .zip file, I have a folder called "Atlas_v1.10.3" on my desktop. If I were to move this to my Addons folder, the addon would not function, as the World of Warcraft client would not have proper access to the addon files. Opening this folder reveals the following sub-folders:
Now that you have placed the appropriate folders into your Addons directory, you can go ahead and launch World of Warcraft. You will need to login, but stop at the Character Selection screen. In the lower left-hand corner of this screen, you will see a AddOns button. Clicking this button will open an AddOns List interface. At this screen, select the "Load out of date AddOns" option. From this interface, you can select specific addons to be loaded for individual characters. This can come in handy if, like me, you only use addons like Auctioneer on a single "bank character". Click "Okay", and you're all set.
Once you enter the game world you will need to configure your addons in whichever way you choose. Congratulations, successfully navigated basic addon installation. Stay tuned for more about addons from Addon Spotlight and check out the WoW Rookie feature for more beginner guides.