I have recently tried (yet again) to make some bootable USB keys on a mac. Turns out you have to convert iso to UDxx:

    $ hdiutil convert -format UDRW -o bootable.img bootable.iso && dd if=bootable.img of=/dev/disk1 bs=1m # Check the disk number with diskutil list!

If it still won't boot, rEFIt (or Boot Camp, naturally) help with BIOS emulation.