How to boot from the DVD
I noticed that the question was not answered.
1. Insert OS X install disk.
2. Restart the iMac. At the chime, hold down the OPTION key
3. When presented with the choice of boot disk, choose the DVD
An alternate method would be to insert the disk, restart and hold down the "C" key until you see the spinning gear.
insert the disk, open System preferences and open the Startup disk preference pane. Choose the DVD as the start up disk