It is possible that this new Apple SuperDrive is not compatible with older iMacs.
It was designed to work with the new Mac Mini and, probably, the brand new iMacs.
Return it for a refund.
There are many third party optical drives that will work with older Macs and iMacs.
Take a look here
Apple's USB SuperDrive only works with Apple Systems that do not have a built in SuperDrive.
Please read the System Requirements at > Apple USB SuperDrive - Apple Store (U.S.)
You should return it and get either a LG, LaCie or OWC External DVD Drive.