MotionBuilder recognizes all audio hardware connected to the computer and displays it in the Audio folder.
Select your audio device in the Scene browser Audio folder.
Activate the Online option to start communication with the Audio device when you are ready to start recording.
Once Online is activated, the audio input displays in the Record area.
Test the levels and settings before you click Record.
Activating Online does not start the recording process, it simply makes MotionBuilder open the connection to the selected audio hardware.