[ubuntu] Cant boot linux installer on macbook?

I downloaded ubuntu 14.0.1 LTS for amd64 + mac, and put it on a usb sitck, but when i hold the option key down to get to the boot screen, it only shows mac os, windows and the recovery disk. How do i boot this?







Mid 2007 Macbook, Core 2 duo 2GHz, Mac OSX 10.7, 4GB of RAM, 250GB 7500 RPM HDD