Create bootable USB of Windows 8.1 on Mavericks 10.9.4
You have to have a version of Bootcamp AND Mac that explicitly supports Windows 8.1. Apple has a knowledge-base article that shows what O/S and Mac supports what version of Windows:

http://support.apple.com/kb/HT5634

Unfortunately there are some Mac models that just won't support certain versions of Windows. Interestingly (or oddly..) enough the new MacPro and most (many?) of the new Macs only support windows 8.