Instructions
Go to the Curse website and navigate to the page of the add-on you want to install. Click "Download Now" on the add-on's Web page. When prompted, save the file to your computer.
Double-click the file after it has finished downloading to open it in your ZIP extractor. Extract the files to your desktop; they will be extracted into a folder. Right-click the extracted folder and select "Cut."
Click "Start," "Computer," "C:" and "Program Files." Open the folder containing your game files. Locate the add-ons folder. In Blizzard games, it is commonly found in the Interface folder. Consult the game's manufacturer if you can't locate the add-ons folder; depending on the game, it may be in a different location or have a different name.
Press "Ctrl" and "V" to paste the add-on into the folder. When you restart the game, the add-on or mod you downloaded will be available to play.