The HDMI port on the Denon 3700 which is labeled eARC is an OUTPUT. That is the connection that goes to your TV.
The computer must be connected to one of the INPUTS of the Denon.
This may have just been a communications issue, and you may be doing it properly, but you must NOT connect the OUTPUT of the Denon to the OUTPUT of the computer. That will never work.
MAC (HDMI OUT) --> DENON Input 1-7 (pick one)
DENON (HDMI OUT) (Monitor ARC/eARC) --> TV HDMI IN (ARC/eARC)