By default (for Macs) you will be look in:
Macintosh HD/Applications/World of Warcraft/Interface/Addons/
If the
Interface and
Addons folders are missing you'll have to first create them.
P.S. I don't know what program is the screenshot manariel linked from but probably it's the reason for his confusion (and mine as well since I have not seen anything like that on a mac - unless they changed the Finder to look like that).
P.P.S. If you need a decent free software for working with archives I can recommend:
Ez7z
http://www.macupdate.com/app/mac/19139/ez7z
Keka
http://www.kekaosx.com/en/