Apple doesn't support booting Linux via USB or Linux at all. That may be your problem. You could try booting an OS X installation via USB.
Oh I see what your saying: You CAN boot OS X off of a USB stick, usually not linux though, although some people have been able to do it. I have not yet tried OS X, though. Thanks that makes more sense I figured I wouldn't be able to do it. I'll give it a shot as soon as I can!