What year, screen size, CPU speed and amount of RAM installed in your iMac?
If your iMac is not a mid-2007 iMac model or newer, you can't install OS X 10,8 Mountain Lion.
Tthe highest OS X version that can be installed is OS X 10.7 Lion.
You need to call Apple at 1-800-MYAPPLE ask for retail software sales and tell them you want to install OS X 10,7 Lion onto your iMac. They will give you a special download code to access the OS X 10.7 Lion download from the Mac App Store.