Most WoW Addons must be manually installed into the correct folder in order for them to work. They are all different but they all have a similar process to do this.
First of all, download your desired addon. They almost always come in zip folders. After saving it, you need to extract it onto your desktop.
Once extracted, find the main folder of the addon. This will be the folder with the addon's name. Each addon comes packaged differently. If it has a lot of files but no folder with its name, make a new folder with the name of the addon, then put all the files into it. If it comes in a folder marked addons or interface, the folder you are looking for is probably inside these. Some addons come with multiple addon tools, each with its own folder. Keep these in their own folders with their own names.
Now you need to find the folder you will put them in. Click on "My Computer", then go to your hard drive (Usually marked C or D), then open the "Program Files" folder, and go to the "World of Warcraft" folder. Go into the "Interface" folder and open the "Addons" folder.
Make sure WoW is not running. Then copy the folder containing your addon and paste it into the "Addons" folder. Now start up WoW. On the character select screen, click the Addons button in the bottom left and make sure your new addon shows up there. If it does, make sure it is checked to be enabled. Enter the game and your addon should be working.
They have different menus and commands but most of them will show a little circle next to your minimap so you can use them. If you can't figure out how to use your addon, just go to the page you downloaded it from and read up on the instructions.
If your addon doesn't work, check to make sure the addon is in the right folder. Open up the folder your addon is in and make sure the files for it are right inside the folder. If there is another folder inside the folder with your addon's title, move the files into the main folder. You want WoW to find the files at World of Warcraft>Interface>Addons>(Your New Addon)>(The files of your new addon).
If you have any questions about any addon posted on these forums, feel free to reply to the post and its author may be able to answer your questions.