macOS should ask when HQPlayer is being started for the first time if HQPlayer is allowed to accept incoming network connections.
But if “localhost” is not working for you on Mac, you can as well type 127.0.0.1 there, it has the same effect.
Also remember to check that the “Allow network control” button is pressed. (shouldn’t be needed for localhost connections though)
Another item to check is if HQPlayer complains on the status bar that it cannot access internet.