For the Mac, you are putting them in the correct folder, so that's no worries. However, you do need to make sure that they have been extracted/unzipped into their own folder within the Addons folder.
So you would see something like:
Interface
...Addons
......Blizzard_AuctionUI
......EquipCompare
There's also those addons that have several folders within their zipped folder, like Deadly Boss Mods. For those, you need to extract/unzip the original folder onto your desktop, open it to see a bunch of sub folders, grab the sub folders and move -those- into the Addons folder within WoW.
How do you tell the difference? Well when you open it, if you see loose files, some saying ".lua" at the end, that should be an addon folder ready to be put into the Addons folder. If you see a bunch of subfolders with no loose files (other than a ReadMe), then those subfolders are what you move into the Addons folder.
Hope that helps.