As someone who plays WoW on a Mac under Mac OS X, it is basically the same as on Windows on a PC. The only main difference, is that some AddOn authors make self-extracting archive programs that will install AddOns for you, but they only run on Windows (they end in .exe).
If you know where your World of Warcraft folder is, look for an Interface folder inside that. Inside the Interface folder should be an AddOns folder. You should not have to create the AddOns folder as there should already be standard ones in there starting with "Blizzard_" in the their names.
To install an AddOn, make sure it is unzipped or uncompressed and appears as a folder, like "QuestAnnouncer" or something like that, and put it in the AddOns folder.
Make sure you are putting the correct AddOn folder in the AddOns folder. Some AddOns are packaged together, like Atlas which has a bunch of parts. To tell a real AddOn folder look inside it for a file that ends in .toc (like QuestAnnouncer.toc).
Once you have the AddOn folder in the AddOns folder, you're done! Launch WoW, log in, click one of your characters (don't click Enter World yet), and click the AddOns button in the lower left corner. If your AddOn has been correctly installed, it should show up in the list with a checkbox and no error to the right. Sometimes AddOns are out-of-date.