[Fizzlemizz]

If you can't see the Addon button it means you don't have any addons installed (correctly).

Usually an addon comes in a .zip file with the addon name and version or date or... ie.
SomeAddon 106.zip.

Most people will unzip this to a folder called:
SomeAddon 106

Inside this will be a folder called:
SomeAddon
There may be more than one folder.

Completely exit the game and:
Copy the SomeAddon (and any other) folder(s) to your [drive]:\[Program Files]\World of Warcraft\Interface\Addons
folder (There will already be a bunch of folders starting with Blizzard_).

Start the game and you should have the Addons button, bottom left of the character select screen. Click it.

If the addon is not selected it is most likely out-of-date. Check the Load Out-of_date Addons box.

Press ok. and start the game. The Addons download page should have usage and or configuration information if it's required.