How do I boot from DVD to do clean install of Snow Leopard on iMac4,1?
You need to press and hold the key(s) immediately after you turn on your Mac and hear the startup sound. Keep holding until the described behavior occurs. To Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media), press and hold the C key.  Or try using the Startup Manager to see if the DVD is available as a boot device, press and hold the Option key.  See: Startup key combinations for Mac

Also have a look at: How to choose a startup disk on your Mac