There is a small folder inside the minecraft/bin folder called natives. This folder contains files that have platform-specific code. When you copied over your install, you copied over the Windows-native files in this folder. As a result, the game fails to run.
You can fix this by replacing the contents of that folder with the correct files from a valid 1.4.6 installation that was done on a Mac.
I'm not entirely certain how you can do that, but the files from a 1.4.7 install may work if in fact Mojang have not changed very much except for some classes in the main jar file. Archive your modded install somewhere else, install MC 1.4.7 fresh, and save the natives folder it generates somewhere convenient. Wipe the rest and put your modded install back in place, remove its Windows-based natives folder, and drop in your new one. See if it runs.
If using a 1.4.7 natives folder doesn't work though, one way to try would be to use Minecraft Nostalgia, which can downgrade the latest version of Minecraft to an arbitrary previous version. (I've used it for downgrading to 1.2.5 for modding purposes, but I haven't done it on a Mac. It appears to support Win/Mac/Linux though.)
You would safely archive your modded install, wipe it from the minecraft/ folder, then run the vanilla launcher to get 1.4.7. Close it out, then run Minecraft Nostalgia, choosing 1.4.6 from the list. Once it's done the conversion, grab the minecraft/bin/natives folder for later. Wipe the rest, put your modded install in, and delete the minecraft/bin/natives folder that came from Windows. Finally, drop in your natives/ folder gleaned from MC Nostalgia, cross your fingers, and launch!
Again, I've never had to do this, but it should work.
Download wget for windows, I installed to c:\GnuWin32\
Create a directory where you want the files to go, You'll have to move them later
open a command prompt box by clicking start and running cmd.exe
Save the following batch file ( http://pastebin.ca/2476877 ) to a file called (yourdirectoryyoujustmade)/getminecraft.bat
in the command prompt type in getminecraft.bat
Move the files into your minecraft resources folder ( look in the debug window )
Worked for me! Kids are off my back now!
Best Answer
I had that problem too for a couple months. It turned out that my account didn't have the permissions to update the launcher, because the launcher is a universal application over all the accounts and the minecraft versions are separate between accounts. If you go to an admin account and install it there, you should be good!
But if you're not, you're going to want to uninstall minecraft and reinstall it.