Alert: you cannot install this software on your computer

I purchased a used iMac on CL. The owner said he had reinstalled the original OS (v10.6.4 and upgraded it to v10.6.8. to remove his personal data. He did not install the Applications disk. The computer works fine. The model identifier is iMac 10.1. He gave me what he said were the original gray Apple-logo Applications and OS dvds. I tried to install the Applications DVD but receive the error " Alert: you cannot install this software on your computer." He says these are the original disks. I'm new to the Mac and would like some help. Thanks.