the disk contains everything you need to run the game. yes your pc has dx11 but it will run the dx9 installer from the disk and update any missing files.
the real problem is that the game isnt compatible with windows 8 not the direct x version. some have managed to get it to work by setting the game to run in compatibility mode with windows xp sp3 (right click, properties, compatibility, check run in compatibility mode and select windows xp sp3 from the drop down. apply. check run as admin. apply and ok.) double click the icon as normal and the game should run.
how stable it will be is any 1s guess but reports say it does work.