Addons are bits of software that slot into the game and are designed to aid you in playing the game. Some may just be visual, some are functional, some are both. Written by people who enjoy doing that kind of thing.
When you get to your character screen, on the bottom right there should be a button saying "Addons". Click it and it will show what addons you have installed. Check to see it is ticked in this dialog. If not, tick it and that should be it.
If you have already done that...which addon did you download? It may have a particular way of showing itself to you. Check the explanation on it's download page.
If still confused, pass some more details back here.