Headphone processing is enabled in the Renderer Preferences (Mac) or Settings (Windows).    This is also where you choose if the headphone output will be a stereo output or binaural.   The audio output device outputs used for the headphone are set in the Room Setup window Routing page.    There is also a headphone only mode enabled from the Preferences or Setting Driver tab that lets you use another set of outputs and turns off the speaker outputs when engaged.    If using the send/return plugins with the Dolby Production Suite, the return plug-in can be assigned to Headphone L and R.   Assign two returns to the headphone and connect to physical outputs via the I/O to your headphone amp to built-in outputs.