Shown below is a basic guide for installing WOW addons. It should give you some idea of what to do or where you are going wrong.

1. Make sure to exit the game.
   
   
2. Browse to the folder where you installed World of Warcraft in your hard-drive which usually is C:\Games\World of Warcraft
   
   
3. Once there browse to the ..\Interface\Addons\ folder, you will see any addons you currently have listed here. If you want to remove them just delete their folder.
   
   (click image to enlarge)
   
4. When you have downloaded your new chosen addon unzip it and place it inside the Addons folder (see image above on how it should look).
   Example of correct structure "wow/interface/addons/Titan" with files inside
   Exmaple of bad structure "wow/interface/addons/Titan/Titan" with files inside
   
5. When you have done this start wow and login to the character select screen.
   
6. When you can see your alt, you should also see a button on the lower left of the screen called "Addons", you will only see this button if you have installed your addons correctly.
   
   (click image to enlarge)
   
7. Click the Addons button to load the next screen that will list all the addons you have installed, on this screen you can turn the addons Off and On by ticking the boxes next to them.
   
8. When you have selected all the ones you want you need to pay attention to the amount of memory allocated to addons. I would increase the default value as the game may crash if the addons use too much (65 is default, mines on 80mb currently).
   Next you need to tick the box called "load out of date addons", sounds scary but dont worry about it.
   
   (click image to enlarge)
   
9. Press OK, then Enter realm and you should now be able to use your addons.
   Enjoy.