HDMI and USB are different ports with different connectors and pin-outs and I don't know of an adapter to match them up.
If the home cinema system is a new one, it should also have a USB connection and the manual will list the devices that are compatible with it, usually small capacity hard-drives, set-top-box, games machine, etc. so I don't think you will achieve anything by plugging you Mac in there.
Your Mac, you didn't specify laptop or desktop, came with a video-out port (the icon on the port looks like a square in brackets) and an adapter that plugs into this port.
The other end of the adapter takes a standard PC VGA cable. Attach this to the other end of the Mac adapter and attach this directly to your TV's PC port and start you Mac. In effect you will be using your TV as an additional monitor.
As VGA isn't designed to carry audio, you will need to run a cable from the headphone socket on the Mac to an audio in port on either the TV or the home cinema system.
Happy viewing on your surround sound system.
HTH.