Due to Apple's new OSX update, they have moved where Application support previously was. During updating/force updating Minecraft says that <User>/Library/Application Support/Minecraft/bin/minecraft.jar cannot be found. I have not found a way to fix this myself. I recommend 2 downloads per OSX version on the Minecraft website or a patched launcher for the newer OSX versions.
Note that I was on my friend's iMac and I cannot dertemine the exact version, but I think the OSX version was 10.8.2.
Thanks
-Cameron
I can confirm what Ryan said. As of Lion, the Library folder is hidden.
Read more here: http://gothick.org.uk/2011/07/19/revealing-lions-hidden-library-folder/