The Audio MIDI Setup is now embedded into the operating system. You can't download it anymore. Hoever, if you have issues with it, connect to the Internet and then go to Mac Logo > Software Update. Check for updates and wait for its installation. Alternatively, remove the software/driver and then use the update function again and it should be installed to the latest version. However, there is a possibility the software you're using has compatibility issues with the driver and because of that you encounter problems.