The steps with the website should still work, but your replays are at a different place. Here are some instructions I was given.
Replay Directory Path: Machintosh HD/Users/"your username here"/Library/Application Support/Blizzard/Starcraft II/Accounts/######/#-S2-#-#######/Replays/Unsaved/Multiplayer/
Note to users of Mac OS X 10.7(Lion) and Max OS X 10.8(Mountain Lion):
The library folder is hidden in Finder by default; to un-hide it, open the terminal and copy and paste in "chflags nohidden ~/Library/" without the quotation marks and press enter. The Terminal application is located in the "\Applications\Utilities\" directory. Alternatively, you can use Spotlight (like a real human being) and simply press Command-Space and type "Terminal" then hit Enter.
Another way to get to the StarCraftII folder is to use the "Go To Folder" feature.
Open Finder and click "Go" on the top, scroll to "Go To Folder..." and open it. Copy paste in "~/Library/Application Support/Blizzard/StarCraft II/" without the quotation marks.
Then go into \Accounts\#######\#-S2-1-#######\Replays\Unsaved\Multiplayer