I have Ubuntu 12.04 as a single install on a MacBook Air 13" (2011 model). I want to try other Linux distributions from a DVD. My portable DVD writer (Samsung SE-208) is plugged into the USB drive, there's a DVD of Linux distributions in the tray, and I've read the invitation page on the DVD which seems okay. However, when I reboot the laptop, whether as a restart or a switch-off-switch-on, instead of booting from the DVD, it just goes into Ubuntu each time. It didn't do that when I switched from Mint to Ubuntu.

How can I get the laptop to boot from the DVD?