Make sure in Options-Midi Devices that the MIDI port you have connected to your BRC is highlighted as an in and out port.
Go to Options, Project, Clock, set source to SMPTE/MTC.
Then go to Midi Out, checkmark Transmit MMC.
The first step will tell Sonar to slave to the incoming timecode. The second will tell Sonar to start the BRC when you push play.