Ah, the joys of Addons.
If you want to make life easy on yourself Id suggest
http://www.wowace.com/wiki/WowAceUpdaterIts an external program that lists all the Ace addons. Ace is a framework on which some of the most popular addons are based.
The best thing about this program is, you dont have to mess about with slow websites (patch day is always a nightmare for UI sites) or unzipping folders and moving stuff around.
Tick the box of the addon you want, and go to the menu and click install, done! (Remember not to have WoW running at the time).
Some of my favourite addons are external to Ace, but Im slowly replacing all independant mods with the Ace equivalent.
www.ctmod.net is a good start, the Core package should get you on your way. As always feel free to ask more questions.