Step 1: Make a Folder to Save the Addons In!
make a folder called "my mods" or whatever you like. this folder will be used to save your addons in.
Step 2: Find the Addon!
go to http://wow.curse.com/downloads/addons/ and search for what addon you want to get.
Step 3: Download Your Addon
click the download button
then click choose mirror automatically and save to the folder you made earlier.
Step 4: Get Them in Your WoW Folder!!
now open your wow folder and then click the interface folder. inside that will be a folder called addons. copy and paste the addons you downloaded earlier into this folder.
Step 5: Go on Wow and Make It Work!!
log on to wow and click the button that says "addons". then click "load out of date addons" and make sure that the addons you downloaded are checked off.