the gist of the answer is press the option key during boot and select the device....some devices have issues. there are troubleshooting steps at the website linked above
A single-filesystem image like the Ubuntu amd64+mac ISO or Debian installer ISOs on a USB stick will work fine to boot MacBooks from the boot selector that appears when you press the Alt key during startup.
solution on the apple support site: http://support.apple.com/kb/HT1948
the gist of the answer is press the option key during boot and select the device....some devices have issues. there are troubleshooting steps at the website linked above
Just see the link below.. this might tell you Choose your device carefully https://help.ubuntu.com/community/MacBook
A single-filesystem image like the Ubuntu amd64+mac ISO or Debian installer ISOs on a USB stick will work fine to boot MacBooks from the boot selector that appears when you press the Alt key during startup.