imessages not recognizing imac

After upgrading to the latest iOS 9 on my iPhone 6S Plus and installing El Capitan on my iMac, I can no longer send or receive messages from my iMac. My iPhone Messages app only gives me the option to forward messages to another device I own but it does not recognize the iMac. (Oddly, I can sync my iPhone with my iMac but, as mentioned, it does not recognize it in the Messages settings.) I've tried virtually every fix suggested on line (rebooting, turning various settings on and off, etc.) but without success.

Any suggestions would be appreciated.

Gary