Did you try starting up with the Windows 7 Disk in the DVD?
If Windows Installation DVD (or Bootable USB) is connected to your iMac you should be able to start up and hold down the OPTION key at the chime.
The DVD (or Bootable USB) Icon should eventually appear on the startup screen.
Select the DVD (or Bootable USB) Icon and start from there.
The Windows 7 Installation should start up and you can then select the BOOTCAMP Partition where to install Windows 7.