Command + R not working on Snow Leopard reboot
Command+R is used to enter Recovery Partition which comes with Lion and Mountain Lion.

To start up from the installation DVD (or any bootable CD or USB, for that matter) you must reboot holding down C before the chime and hold until the Apple logo appears.

You could, too, just let it boot normally, and then run the installer (once the DVD is mounted in Finder), which will, then, start up into the DVD.