If you want addons and you need the in the TBC version, continue on.
Step By Step;
Go to curseForge.com
Search ANY addon in the searchbar in the top right hand corner.
Once selected, click the 'Files' tab. As illustrated below.
After that, find the version of the game you desire. ( In our case, It is 2.4.3 ) As illustrated below.
Download that version. Make sure it's a release and not a 'Beta' or 'Alpha'.
After it's downloaded, extract the file into Hardrive > Program Files > World of Warcraft > Interface > Addons.
Then in game, look to your bottom left corner. Click that.
Select the addons you want to enable, and then go ingame and configure them. :)
BY THE WAY:
Not EVERY addon will be available. BUT most of the popular ones were made before 2.4.3.
I'm Trozzy, and I approve this message. :)