cd ~
mkdir mctmp
cd mctmp
jar xf ~/Library/Application\ Support/minecraft/bin/minecraft.jar

Copy that into Terminal, then press enter. DO NOT CLOSE TERMINAL. Go to Finder. Click on your username and you will see a folder called mctmp this is where all of the minecraft files are stored. Change whatever you want to change, then go back to terminal and paste in this:

rm META-INF/MOJANG_C.*
jar uf ~/Library/Application\ Support/minecraft/bin/minecraft.jar ./
cd ..
rm -rf mctmp

Press enter once more, you will notice the mctmp folder is gone. You can now close terminal. Hope I was able to help you!