Won't boot from DVD

[Rutilus]

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?

[2F4U]

Do you press the alt key so that you get the boot menu?

[Rutilus]

Thank you - that's what I need to do.

[2F4U]

You are welcome. I stumbled about this when I first tried to install to my MacBook.