1. Locate Minecraft.
2. Right click on it, Show Package Contents.
3. Descend into the "Contents" directory and locate Info.plist.
4. Right click, Open with TextEdit.
5. Scroll all the way down and look for this line:
You have probably written an invalid value. When i tried to change it to 0,5G I got the same problem, and switching it back to the default values fixed it. If that doesn't work it is possible that you have screwed up some other part of the info.plst. In that case you'll have to get a fresh one somewhere. (Maybe you can delete it and it will create a fresh one for you?) Back your stuff up though!
If non of this work, you could always re-install Minecraft. Ans as always remember to backup everything of importance!