The installer and some of the software is tied to OS version. So eventhough my 2013 LT from the mac app store will run on yosimite (10.10) I can't install it be cause the installer won't allow it to be installed on anything but 10.6 to 10.9. This is a problem I see only getting worse as a new OS comes out every year and the software is tied to OS identification rather than OS functionality and design.
Another example of this would be AC LT 2012. A co worker has it on his Mac. it worked great on 10.8. It had errors on 10.9. He upgraded his 10.8 mac to 10.10 and it worked fine again.
So sadly your best course of action is to upgrade your OS. Or find a "legacy" copy of an older, compatable AC for MAC. However, if youy have the ability, $$ and a powerful enough machine, I would get VMware Fusion, Windows, and autocad for windows. it costs more but is better software.