Your device doesn't see the boot files because your USB drive is not formatted correctly. Your legacy boot can only read FAT16. Your Mac automatically formats FAT systems with FAT32. To format in FAT16, you have to go to your Mac command line and use this script: diskutil partitionDisk path MBR "MS-DOS FAT16" "name" 100%
path is the path to the device (/dev/disk1). MBR impliments the Master Boot Record format. Obviously, next is the file system to be used (must be enclosed in quotes). name is the name to be given to the drive (also must be enclosed in quotes). 100% says the entire drive is to be used. Keep in mind, FAT16 is limited to 2GB total.
Or you could just find yourself a Win machine, it formats to FAT16 quite easily. But if you want to do it sweet style, use your command line. Good luck.