You can't install from the Snow Leopard retail DVD (which is 10.6.3) on any iMac newer than March 2010.
Ref. A.Brodie's article -
If you can obtain the grey discs that would have shipped with the Mac had Lion not been installed (try Apple Customer Services with the serial no. of the Mac and a credit card handy), you may be able to install from that (10.6.7)
Otherwise there may be a workround, but I can't personally vouch for its efficacy.
It involves installing 10.6.3 on an external drive, updating to 10.6.8 with the combo installer, boot from the external drive and clone the Snow Leopard onto your partition.
The installation to an external drive may need to be done from another Mac running SL.
That's just a précis, and as I said, I've never tried it personally, so research it a little more.