I'm having the same problem. The first time I tried to open a modpack, it got to the Mojang screen but then quit after a few seconds. After that, it never got to that point at all, it would show FML loading and then everything would quit. I tried deleting and re-downloading, but nothing has changed. I have a MacBook Pro with version 10.8.3 installed. Not sure what the java version is, but any help would be greatly appreciated.