The new iMac doesn't come with an optical drive, right? And, from what I've read, the Windows installer doesn't play well with external disc drives. Best course of action may be to have Boot Camp Assistant create a bootable Windows thumb drive.
Insert your Windows disc and open Disk Utility. Click
New Image. On the window that pops up, choose
DVD/CD master as the file format and click ok. (This will create a .cdr file.)
Then open Boot Camp Assistant and check the box beside
Create a Windows 7 install disk.
You may need to change the .cdr extension to .iso, but I think BCA will recognize the file as is.